xfs: xfs_rtbuf_get should check the bmapi_read results

The xfs_rtbuf_get function should check the block mapping it gets back
from bmapi_read.  If there are no mappings or the mapping isn't a real
extent, we should return -EFSCORRUPTED rather than trying to read a
garbage value.  We also require realtime bitmap blocks to be real,
written allocations.
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>

fs/xfs/libxfs/xfs_rtbitmap.c

@@ -90,6 +90,9 @@ xfs_rtbuf_get(
 	if (error)
 		return error;
 
+	if (nmap == 0 || !xfs_bmap_is_real_extent(&map))
+		return -EFSCORRUPTED;
+
 	ASSERT(map.br_startblock != NULLFSBLOCK);
 	error = xfs_trans_read_buf(mp, tp, mp->m_ddev_targp,
 				   XFS_FSB_TO_DADDR(mp, map.br_startblock),