libata: don't use on-stack sense buffer

sense_buffer is used as DMA target and shouldn't be allocated on
stack.  Use ap->sector_buf instead.  This problem is spotted by Chuck
Ebbert.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>