cifs: Fix cifs_write_back_from_locked_folio()

cifs_write_back_from_locked_folio() should return the number of bytes read,
but returns the result of ->async_writev(), which will be 0 on success.  As
it happens, this doesn't prevent cifs_writepages_region() from working as
it will then examine and ignore the pages that are no longer dirty rather
than just skipping over them.

Fixes: d08089f ("cifs: Change the I/O paths to use an iterator rather than a page list")
Signed-off-by: David Howells <[email protected]>
cc: Shyam Prasad N <[email protected]>
cc: Rohith Surabattula <[email protected]>
cc: Tom Talpey <[email protected]>
cc: Jeff Layton <[email protected]>
cc: [email protected]
Reviewed-by: Paulo Alcantara (SUSE) <[email protected]>
Signed-off-by: Steve French <[email protected]>

fs/cifs/file.c

@@ -2845,6 +2845,7 @@ static ssize_t cifs_write_back_from_locked_folio(struct address_space *mapping,
 	free_xid(xid);
 	if (rc == 0) {
 		wbc->nr_to_write = count;
+		rc = len;
 	} else if (is_retryable_error(rc)) {
 		cifs_pages_write_redirty(inode, start, len);
 	} else {