dax: pass NOWAIT flag to iomap_apply

fstests generic/471 reports a failure when run with MOUNT_OPTIONS="-o
dax".  The reason is that the initial pwrite to an empty file with the
RWF_NOWAIT flag set does not return -EAGAIN.  It turns out that
dax_iomap_rw doesn't pass that flag through to iomap_apply.

With this patch applied, generic/471 passes for me.

Signed-off-by: Jeff Moyer <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Dan Williams <[email protected]>

Author: JeffMoyer

fs/dax.c

@@ -1201,6 +1201,9 @@ dax_iomap_rw(struct kiocb *iocb, struct iov_iter *iter,
 		lockdep_assert_held(&inode->i_rwsem);
 	}
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		flags |= IOMAP_NOWAIT;
+
 	while (iov_iter_count(iter)) {
 		ret = iomap_apply(inode, pos, iov_iter_count(iter), flags, ops,
 				iter, dax_iomap_actor);