MD: not clear ->safemode for external metadata array

->safemode should be triggered by mdadm for external metadaa array, otherwise
array's state confuses mdadm.

Fixes: 33182d15(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>