[SCSI] iscsi transport: use atomic for session_nr allocations

qla4xxx and iscsi_tcp or iser could be creating
sessions at the same time, so make session_nr id
allocation atomic.

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