[SCSI] tgt: set the data length properly

scsi_tgt uses REQ_TYPE_BLOCK_PC so scsi_init_io doesn't set the length
for us. scsi_tgt needs to do it by itself.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>