xfs: Check for extent overflow when remapping an extent

Remapping an extent involves unmapping the existing extent and mapping
in the new extent. When unmapping, an extent containing the entire unmap
range can be split into two extents,
i.e. | Old extent | hole | Old extent |
Hence extent count increases by 1.

Mapping in the new extent into the destination file can increase the
extent count by 1.

Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_reflink.c

@@ -1006,6 +1006,7 @@ xfs_reflink_remap_extent(
 	unsigned int		resblks;
 	bool			smap_real;
 	bool			dmap_written = xfs_bmap_is_written_extent(dmap);
+	int			iext_delta = 0;
 	int			nimaps;
 	int			error;
 
@@ -1099,6 +1100,16 @@ xfs_reflink_remap_extent(
 			goto out_cancel;
 	}
 
+	if (smap_real)
+		++iext_delta;
+
+	if (dmap_written)
+		++iext_delta;
+
+	error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK, iext_delta);
+	if (error)
+		goto out_cancel;
+
 	if (smap_real) {
 		/*
 		 * If the extent we're unmapping is backed by storage (written