netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry

netfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry

When a write subrequest is marked NETFS_SREQ_NEED_RETRY, the retry path
in netfs_unbuffered_write() unconditionally calls stream->prepare_write()
without checking if it is NULL.

Filesystems such as 9P do not set the prepare_write operation, so
stream->prepare_write remains NULL. When get_user_pages() fails with
-EFAULT and the subrequest is flagged for retry, this results in a NULL
pointer dereference at fs/netfs/direct_write.c:189.

Fix this by mirroring the pattern already used in write_retry.c: if
stream->prepare_write is NULL, skip renegotiation and directly reissue
the subrequest via netfs_reissue_write(), which handles iterator reset,
IN_PROGRESS flag, stats update and reissue internally.

Fixes: a0b4c7a49137 ("netfs: Fix unbuffered/DIO writes to dispatch subrequests in strict sequence")
Reported-by: syzbot+7227db0fbac9f348dba0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7227db0fbac9f348dba0
Signed-off-by: Deepanshu Kartikey <Kartikey406@gmail.com>
Link: https://patch.msgid.link/20260307043947.347092-1-kartikey406@gmail.com
Tested-by: syzbot+7227db0fbac9f348dba0@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>