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>