netfs: Fix kernel async DIO

Netfslib needs to be able to handle kernel-initiated asynchronous DIO that
is supplied with a bio_vec[] array.  Currently, because of the async flag,
this gets passed to netfs_extract_user_iter() which throws a warning and
fails because it only handles IOVEC and UBUF iterators.  This can be
triggered through a combination of cifs and a loopback blockdev with
something like:

        mount //my/cifs/share /foo
        dd if=/dev/zero of=/foo/m0 bs=4K count=1K
        losetup --sector-size 4096 --direct-io=on /dev/loop2046 /foo/m0
        echo hello >/dev/loop2046

This causes the following to appear in syslog:

        WARNING: CPU: 2 PID: 109 at fs/netfs/iterator.c:50 netfs_extract_user_iter+0x170/0x250 [netfs]

and the write to fail.

Fix this by removing the check in netfs_unbuffered_write_iter_locked() that
causes async kernel DIO writes to be handled as userspace writes.  Note
that this change relies on the kernel caller maintaining the existence of
the bio_vec array (or kvec[] or folio_queue) until the op is complete.

Fixes: 153a996 ("netfs: Implement unbuffered/DIO write support")
Reported-by: Nicolas Baranger <nicolas.baranger@3xo.fr>
Closes: https://lore.kernel.org/r/fedd8a40d54b2969097ffa4507979858@3xo.fr/
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/608725.1736275167@warthog.procyon.org.uk
Tested-by: Nicolas Baranger <nicolas.baranger@3xo.fr>
Acked-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
cc: Steve French <smfrench@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: dhowells

fs/netfs/direct_write.c

@@ -67,7 +67,7 @@ ssize_t netfs_unbuffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *
 		 * allocate a sufficiently large bvec array and may shorten the
 		 * request.
 		 */
-		if (async || user_backed_iter(iter)) {
+		if (user_backed_iter(iter)) {
 			n = netfs_extract_user_iter(iter, len, &wreq->iter, 0);
 			if (n < 0) {
 				ret = n;
@@ -77,6 +77,11 @@ ssize_t netfs_unbuffered_write_iter_locked(struct kiocb *iocb, struct iov_iter *
 			wreq->direct_bv_count = n;
 			wreq->direct_bv_unpin = iov_iter_extract_will_pin(iter);
 		} else {
+			/* If this is a kernel-generated async DIO request,
+			 * assume that any resources the iterator points to
+			 * (eg. a bio_vec array) will persist till the end of
+			 * the op.
+			 */
 			wreq->iter = *iter;
 		}