iov_iter: Fix a memory leak in iov_iter_extract_user_pages()

iov_iter: Fix a memory leak in iov_iter_extract_user_pages()

There's a potential memory leak in callers of iov_iter_extract_user_pages()
whereby if a pages array is allocated in function, it isn't freed before
returning of an error or 0.

Now, it's not a leak per se in iov_iter_extract_user_pages() as, if an
array is allocated, it's returned through *pages, so it's incumbent on the
caller to free it.  However, not all callers do.

Fix this by freeing the table and clearing *pages before returning an error
or 0.  Note that iov_iter_extract_pages() and its subfunctions are allowed
to return 0 without returning an array (for instance if the iterator count
is 0).

Fixes: 7d58fe731028 ("iov_iter: Add a function to extract a page list from an iterator")
Closes: https://sashiko.dev/#/patchset/20260616100821.2062304-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260625140640.3116900-8-dhowells@redhat.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>