netfs: Fix potential uninitialised var in netfs_extract_user_iter()

commit 7e3d8db upstream.

In netfs_extract_user_iter(), if it's given a zero-length iterator, it will
fall through the loop without setting ret, and so the error handling
behaviour will be undefined, depending on whether ret happens to be
negative.  The value of ret then propagates back up the callstack.

Fix this by presetting ret to 0.

Fixes: 85dd2c8 ("netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator")
Closes: https://sashiko.dev/#/patchset/20260414082004.3756080-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260512123404.719402-9-dhowells@redhat.com
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dhowells

fs/netfs/iterator.c

@@ -43,7 +43,7 @@ ssize_t netfs_extract_user_iter(struct iov_iter *orig, size_t orig_len,
 	unsigned int max_pages;
 	unsigned int npages = 0;
 	unsigned int i;
-	ssize_t ret;
+	ssize_t ret = 0;
 	size_t count = orig_len, offset, len;
 	size_t bv_size, pg_size;