iovec should be reclaimed whenever caller of rw_copy_check_uvector() returns,
but it doesn't hold when failure happens right after aio_setup_vectored_rw().

Fix that in a such way to avoid hairy goto.
Signed-off-by: NLeon Yu <chianglungyu@gmail.com>
Signed-off-by: NBenjamin LaHaise <bcrl@kvack.org>
Cc: stable@vger.kernel.org