xfs: check overlapping rmap btree records

The rmap btree scrubber doesn't contain sufficient checking for records
that cannot overlap but do anyway.  For the other btrees, this is
enforced by the inorder checks in xchk_btree_rec, but the rmap btree is
special because it allows overlapping records to handle shared data
extents.

Therefore, enhance the rmap btree record check function to compare each
record against the previous one so that we can detect overlapping rmap
records for space allocations that do not allow sharing.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>