xfs: don't crash if reading a directory results in an unexpected hole

In xfs_dir3_data_read, we can encounter the situation where err == 0 and
*bpp == NULL if the given bno offset happens to be a hole; this leads to
a crash if we try to set the buffer type after the _da_read_buf call.
Holes can happen due to corrupt or malicious entries in the bmbt data,
so be a little more careful when we're handling buffers.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_dir2_data.c

@@ -329,7 +329,7 @@ xfs_dir3_data_read(
 
 	err = xfs_da_read_buf(tp, dp, bno, mapped_bno, bpp,
 				XFS_DATA_FORK, &xfs_dir3_data_buf_ops);
-	if (!err && tp)
+	if (!err && tp && *bpp)
 		xfs_trans_buf_set_type(tp, *bpp, XFS_BLFT_DIR_DATA_BUF);
 	return err;
 }