xfs: strengthen rmap record flags checking

We always know the correct state of the rmap record flags (attr, bmbt,
unwritten) so check them by direct comparison.

Fixes: d852657c ("xfs: cross-reference reverse-mapping btree")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>