powernv/subpage_prot: kernel NULL pointer dereference running kernel self tests (subpage_prot)

[Sachin Sant]

While running kselftests (subpage_prot) ran into following OOPS. 
This is against linuxppc merge branch. Does not happen against mainline.
Same problem is seen with latest linux-next tree as well.

Following recent commits have modified the code in question.

ef629cc5bf0543eb57d6e344ba776880ac35fd00 :
    powerc/mm/hash: Reduce hash_mm_context size

60458fba469a695a026334b364cf8adbcd5807e3: 
   powerpc/mm: Add helpers for accessing hash translation related variables


(47/47) avocado-misc-tests/kernel/kselftest.py:kselftest.test:  [17020.529615] BUG: Kernel NULL pointer dereference at 0x00001038
[17020.529639] Faulting instruction address: 0xc00000000008bdb4
[17020.529646] Oops: Kernel access of bad area, sig: 11 [#1]
[17020.529651] LE PAGE_SIZE=64K MMU=Radix MMU=Hash SMP NR_CPUS=2048 NUMA PowerNV
[17020.529695] Dumping ftrace buffer:
[17020.529779]    (ftrace buffer empty)
[17020.529788] Modules linked in: iscsi_target_mod target_core_mod macsec tcp_diag udp_diag inet_diag unix_diag af_packet_diag netlink_diag binfmt_misc xt_CHECKSUM ipt_MASQUERADE tun ip6t_rpfilter ipt_REJECT nf_reject_ipv4 ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set nfnetlink ebtable_nat ebtable_broute bridge stp llc ip6table_nat ip6table_mangle ip6table_security ip6table_raw iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 libcrc32c iptable_mangle iptable_security iptable_raw ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter kvm_hv kvm i2c_dev sunrpc dm_mirror dm_region_hash dm_log dm_mod ses enclosure scsi_transport_sas sg ibmpowernv ipmi_powernv leds_powernv ipmi_devintf uio_pdrv_genirq ipmi_msghandler uio opal_prd powernv_op_panel ip_tables ext4 mbcache jbd2 sd_mod ipr libata tg3 ptp pps_core [last unloaded: kretprobe_example]
[17020.529873] CPU: 138 PID: 17457 Comm: subpage_prot Tainted: G           O      5.1.0-rc6-autotest-autotest #1
[17020.529880] NIP:  c00000000008bdb4 LR: c00000000000b688 CTR: c00000000008bd40
[17020.529886] REGS: c00020002c6b7aa0 TRAP: 0300   Tainted: G           O       (5.1.0-rc6-autotest-autotest)
[17020.529892] MSR:  9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 22000842  XER: 00000000
[17020.529901] CFAR: c00000000000b684 DAR: 0000000000001038 DSISR: 40000000 IRQMASK: 0 
[17020.529901] GPR00: c00000000000b688 c00020002c6b7d30 c0000000013b6600 0000000000000000 
[17020.529901] GPR04: 0000000000000000 0000000000000000 0000000000000001 0000000000000008 
[17020.529901] GPR08: 0000000000000000 0000000000000000 0000000000000000 c000000000ab1058 
[17020.529901] GPR12: c00000000008bd40 c0002007ff6e5900 0000000000000000 0000000000000000 
[17020.529901] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
[17020.529901] GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
[17020.529901] GPR24: 0000000000000000 0000000000000000 0000000000000000 00007fff997afbb8 
[17020.529901] GPR28: 00007fff997b0000 0000000010001f50 0000000010001350 c000200662c3ea00 
[17020.529949] NIP [c00000000008bdb4] sys_subpage_prot+0x74/0x590
[17020.529955] LR [c00000000000b688] system_call+0x5c/0x70
[17020.529959] Call Trace:
[17020.529964] [c00020002c6b7d30] [c00020002c6b7d90] 0xc00020002c6b7d90 (unreliable)
[17020.529971] [c00020002c6b7e20] [c00000000000b688] system_call+0x5c/0x70
[17020.529976] Instruction dump:
[17020.529980] fb61ffd8 fb81ffe0 fba1ffe8 fbc1fff0 fbe1fff8 f821ff11 e92d1178 f9210068 
[17020.529987] 39200000 e92d0968 ebe90630 e93f03e8 <eb891038> 60000000 3860fffe e9410068 
[17020.529997] ---[ end trace 90c07b8228c575ad ]—

Thanks
-Sachin