md/raid5: fix deadlock that raid5d() wait for itself to clear MD_SB_CHANGE_PENDING

In the Linux kernel, the following vulnerability has been resolved:

md/raid5: fix deadlock that raid5d() wait for itself to clear MD_SB_CHANGE_PENDING

Xiao reported that lvm2 test lvconvert-raid-takeover.sh can hang with

small possibility, the root cause is exactly the same as commit

bed9e27baf52 ("Revert "md/raid5: Wait for MD_SB_CHANGE_PENDING in raid5d"")

However, Dan reported another hang after that, and junxiao investigated

the problem and found out that this is caused by plugged bio can't issue

from raid5d().

Current implementation in raid5d() has a weird dependence:

1) md_check_recovery() from raid5d() must hold 'reconfig_mutex' to clear

MD_SB_CHANGE_PENDING;

2) raid5d() handles IO in a deadloop, until all IO are issued;

3) IO from raid5d() must wait for MD_SB_CHANGE_PENDING to be cleared;

This behaviour is introduce before v2.6, and for consequence, if other

context hold 'reconfig_mutex', and md_check_recovery() can't update

super_block, then raid5d() will waste one cpu 100% by the deadloop, until

'reconfig_mutex' is released.

Refer to the implementation from raid1 and raid10, fix this problem by

skipping issue IO if MD_SB_CHANGE_PENDING is still set after

md_check_recovery(), daemon thread will be woken up when 'reconfig_mutex'

is released. Meanwhile, the hang problem will be fixed as well.