xfs: use ENOTBLK for direct I/O to buffered I/O fallback

This is what the classic fs/direct-io.c implementation and thuse other
file systems use.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/xfs_file.c

@@ -505,7 +505,7 @@ xfs_file_dio_aio_write(
 		 */
 		if (xfs_is_cow_inode(ip)) {
 			trace_xfs_reflink_bounce_dio_write(ip, iocb->ki_pos, count);
-			return -EREMCHG;
+			return -ENOTBLK;
 		}
 		iolock = XFS_IOLOCK_EXCL;
 	} else {
@@ -714,7 +714,7 @@ xfs_file_write_iter(
 		 * allow an operation to fall back to buffered mode.
 		 */
 		ret = xfs_file_dio_aio_write(iocb, from);
-		if (ret != -EREMCHG)
+		if (ret != -ENOTBLK)
 			return ret;
 	}