iomap: Fix direct I/O write consistency check

When a direct I/O write falls back to buffered I/O entirely, dio->size
will be 0 in iomap_dio_complete.  Function invalidate_inode_pages2_range
will try to invalidate the rest of the address space.  If there are any
dirty pages in that range, the write will fail and a "Page cache
invalidation failure on direct I/O" error will be logged.

On gfs2, this can be reproduced as follows:

  xfs_io \
    -c "open -ft foo" -c "pwrite 4k 4k" -c "close" \
    -c "open -d foo" -c "pwrite 0 4k"

Fix this by recognizing 0-length writes.
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/iomap/direct-io.c

@@ -108,7 +108,7 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
 	 * ->end_io() when necessary, otherwise a racing buffer read would cache
 	 * zeros from unwritten extents.
 	 */
-	if (!dio->error &&
+	if (!dio->error && dio->size &&
 	    (dio->flags & IOMAP_DIO_WRITE) && inode->i_mapping->nrpages) {
 		int err;
 		err = invalidate_inode_pages2_range(inode->i_mapping,