ps3disk: passing wrong variable to bvec_kunmap_irq()

This should pass "buf" to bvec_kunmap_irq() instead of "bv".  The api is
like kmap_atomic() instead of kmap().

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>