powerpc/sysrq: Fix oops whem ppmu is not registered

[Michael Ellerman]

On Tue, 2017-09-19 at 06:17:06 UTC, Ravi Bangoria wrote:
> Kernel crashes if power pmu is not registered and user tries to dump
> regs with 'echo p > /proc/sysrq-trigger'. Sample log:
> 
>   Unable to handle kernel paging request for data at address 0x00000008
>   Faulting instruction address: 0xc0000000000d52f0
> 
>   NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
>   LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
>   Call Trace:
>    printk+0x38/0x4c (unreliable)
>    __handle_sysrq+0xe4/0x270
>    write_sysrq_trigger+0x64/0x80
>    proc_reg_write+0x80/0xd0
>    __vfs_write+0x40/0x200
>    vfs_write+0xc8/0x240
>    SyS_write+0x60/0x110
>    system_call+0x58/0x6c
> 
> Signed-off-by: Ravi Bangoria <ravi.bangoria at linux.vnet.ibm.com>
> Reviewed-by: Kamalesh Babulal <kamalesh at linux.vnet.ibm.com>

Applied to powerpc fixes, thanks.

https://git.kernel.org/powerpc/c/4917fcb58cc73f6b81455e3c5f9601

cheers