[SCSI] libiscsi: do not take host lock in queuecommand

iscsi_tcp, ib_iser, cxgb*, be2iscsi and bnx2i do not use
the host lock and do not take the session lock against
a irq, so this patch drops the DEF_SCSI_QCMD use. Instead
we just take the session lock and disable bhs.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>