iov_iter: fix memory leak in pipe_get_pages_alloc()

Make n signed to avoid leaking the pages array if __pipe_get_pages()
fails to allocate any pages.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -1102,7 +1102,7 @@ static ssize_t pipe_get_pages_alloc(struct iov_iter *i,
 		   size_t *start)
 {
 	struct page **p;
-	size_t n;
+	ssize_t n;
 	int idx;
 	int npages;