[SCSI] iscsi class: regression - fix races with state manipulation and blocking/unblocking

For qla4xxx, we could be starting a session, but some error (network,
target, IO from a device that got started, etc) could cause the session
to fail and curring the block/unblock and state manipulation could race
with each other. This patch just has those operations done in the
single threaded iscsi eh work queue, so that way they are serialized.

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