[SCSI] fix use-after-free in scsi_init_io()

we're using a pointer through a freed command to reset the request,
which has shown up as an oops with slab poisoning:

Reported-by: Tejun Heo <tj@kernel.org>
Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>