xfs: reinitialize rm_flags when unpacking an offset into an rmap irec

xfs: reinitialize rm_flags when unpacking an offset into an rmap irec

Source kernel commit: ffb5696f7555e44c04b7c1212fcd83ddedc43498

In xfs_rmap_irec_offset_unpack, we should always clear the contents of
rm_flags before we begin unpacking the encoded (ondisk) offset into the
incore rm_offset and incore rm_flags fields.  Remove the open-coded
field zeroing as this encourages api misuse.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>