md: enhance checking in md_check_recovery()

For md_check_recovery():

1) if 'MD_RECOVERY_RUNING' is not set, register new sync_thread.
2) if 'MD_RECOVERY_RUNING' is set:
 a) if 'MD_RECOVERY_DONE' is not set, don't do anything, wait for
   md_do_sync() to be done.
 b) if 'MD_RECOVERY_DONE' is set, unregister sync_thread. Current code
   expects that sync_thread is not NULL, otherwise new sync_thread will
   be registered, which will corrupt the array.

Make sure md_check_recovery() won't register new sync_thread if
'MD_RECOVERY_RUNING' is still set, and a new WARN_ON_ONCE() is added for
the above corruption,

Signed-off-by: Yu Kuai <[email protected]>
Reviewed-by: Xiao Ni <[email protected]>
Signed-off-by: Song Liu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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