[SCSI] st: add st_scsi_kern_execute helper function

st_scsi_kern_execute is a helper function to perform SCSI commands
synchronously. It supports data transfer with a liner in-kernel buffer
(not scatter gather). st_scsi_kern_execute internally uses
scsi_execute().

The majority of st_do_scsi can be replaced with
st_scsi_kern_execute. This is a preparation for rewriting st_do_scsi
to remove obsolete scsi_execute_async().

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