splice: fix double page unlock

splice: fix double page unlock

If add_to_page_cache_lru() fails, the page will not be locked. But
splice jumps to an error path that does a page release and unlock,
causing a BUG() in unlock_page().

Fix this by adding one more label that just releases the page. This bug
was actually triggered on EL5 by gurudas pai <gurudas.pai@oracle.com>
using fio.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>