md: fix soft lockup in status_resync

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

md: fix soft lockup in status_resync

status_resync() will calculate 'curr_resync - recovery_active' to show
user a progress bar like following:

[============>........] resync = 61.4%

'curr_resync' and 'recovery_active' is updated in md_do_sync(), and
status_resync() can read them concurrently, hence it's possible that
'curr_resync - recovery_active' can overflow to a huge number. In this
case status_resync() will be stuck in the loop to print a large amount
of '=', which will end up soft lockup.

Fix the problem by setting 'resync' to MD_RESYNC_ACTIVE in this case,
this way resync in progress will be reported to user.