xfs: don't allocate into the data fork for an unshare request

For an unshare request, we only have to take action if the data fork has
a shared mapping.  We don't care if someone else set up a cow operation.
If we find nothing in the data fork, return a hole to avoid allocating
space.

Note that fallocate will replace the delalloc reservation with an
unwritten extent anyway, so this has no user-visible effects outside of
avoiding unnecessary updates.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_iomap.c

@@ -1006,8 +1006,9 @@ xfs_buffered_write_iomap_begin(
 	if (eof)
 		imap.br_startoff = end_fsb; /* fake hole until the end */
 
-	/* We never need to allocate blocks for zeroing a hole. */
-	if ((flags & IOMAP_ZERO) && imap.br_startoff > offset_fsb) {
+	/* We never need to allocate blocks for zeroing or unsharing a hole. */
+	if ((flags & (IOMAP_UNSHARE | IOMAP_ZERO)) &&
+	    imap.br_startoff > offset_fsb) {
 		xfs_hole_to_iomap(ip, iomap, offset_fsb, imap.br_startoff);
 		goto out_unlock;
 	}