[SCSI] tgt: fix sesnse buffer problems

This patch simplify the way to notify LLDs of the command completion
and addresses the following sense buffer problems:

- can't handle both data and sense.
- forces user-space to use aligned sense buffer

tgt copies sense_data from userspace to cmnd->sense_buffer (if
necessary), maps user-space pages (if necessary) and then calls
host->transfer_response (host->transfer_data is removed).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>