[powerpc][next-20200701] Hung task timeouts during regression test runs

[Sachin Sant]

Starting with linux-next 20200701 release I am observing automated regressions
tests taking longer time to complete. A test which took 10 minutes with next-20200630
took more than 60 minutes against next-20200701. 

Following hung task timeout messages were seen during these runs

[ 1718.848351]       Not tainted 5.8.0-rc3-next-20200701-autotest #1
[ 1718.848356] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 1718.848362] NetworkManager  D    0  2626      1 0x00040080
[ 1718.848367] Call Trace:
[ 1718.848374] [c0000008b0f6b8f0] [c000000000c6d558] schedule+0x78/0x130 (unreliable)
[ 1718.848382] [c0000008b0f6bad0] [c00000000001b070] __switch_to+0x2e0/0x480
[ 1718.848388] [c0000008b0f6bb30] [c000000000c6ce9c] __schedule+0x2cc/0x910
[ 1718.848394] [c0000008b0f6bc10] [c000000000c6d558] schedule+0x78/0x130
[ 1718.848401] [c0000008b0f6bc40] [c0000000005d5a64] jbd2_log_wait_commit+0xd4/0x1a0
[ 1718.848408] [c0000008b0f6bcc0] [c00000000055fb6c] ext4_sync_file+0x1cc/0x480
[ 1718.848415] [c0000008b0f6bd20] [c000000000493530] vfs_fsync_range+0x70/0xf0
[ 1718.848421] [c0000008b0f6bd60] [c000000000493638] do_fsync+0x58/0xd0
[ 1718.848427] [c0000008b0f6bda0] [c0000000004936d8] sys_fsync+0x28/0x40
[ 1718.848433] [c0000008b0f6bdc0] [c000000000035e28] system_call_exception+0xf8/0x1c0
[ 1718.848440] [c0000008b0f6be20] [c00000000000ca70] system_call_common+0xf0/0x278

Comparing next-20200630 with next-20200701 one possible candidate seems to
be following commit:

commit 37f4a24c2469a10a4c16c641671bd766e276cf9f
    blk-mq: centralise related handling into blk_mq_get_driver_tag

Reverting this commit allows the test to complete in 10 minutes.

Thanks
-Sachin