[SCSI] fix bsg queue oops with iscsi logout

Delay bsg unregistration, because we want to wait until all the request/cmds
have released their reference.

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