->safemode should be triggered by mdadm for external metadaa array, otherwise
array's state confuses mdadm.
Fixes: 33182d15c6bf(md: always clear ->safemode when md_check_recovery gets the mddev lock.)
Cc: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>