[SCSI] libiscsi: use bh locking instead of irq with session lock

The session lock is taken in threads, timers, and bottom halves
like softirqs and tasklets. All the code but
iscsi_conn/session_failure take the session lock with the spin_lock_bh
call. This was done because I thought some offload drivers
would be calling these functions from a irq. They never did,
so this patch has iscsi_conn/session_failure use the bh
locking.

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