iov_iter: fix return type of __pipe_get_pages()

It returns -EFAULT and happens to be a helper for pipe_get_pages()
whose return type is ssize_t.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

lib/iov_iter.c

@@ -1012,7 +1012,7 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
 }
 EXPORT_SYMBOL(iov_iter_gap_alignment);
 
-static inline size_t __pipe_get_pages(struct iov_iter *i,
+static inline ssize_t __pipe_get_pages(struct iov_iter *i,
 				size_t maxsize,
 				struct page **pages,
 				int idx,