[SCSI] tgt: stop zero'ing scsi_cmnd

The scsi midlayer allocates scsi_cmnd->sense_buffer dynamically so we
can't initialize scsi_cmnd (the midlyaer does for us).

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