qla2xxx: Check + fail when npiv_vports_inuse exists in shutdown

This patch adds an check to qlt_stop_phase1() to avoid shutdown
when the base_vha contains a non-zero fc_host->npiv_vports_inuse
count.

This includes holding qla_tgt_mutex in qlt_stop_phase1() between
the fc_host->npiv_vports_inuse check + setting of tgt->tgt_stop to
avoid a possible race between qlt_lport_register() -> tcm_qla2xxx
-> tcm_qla2xxx_lport_register_npiv_cb() calling fc_vport_create().

Cc: Sawan Chandak <sawan.chandak@qlogic.com>
Cc: Quinn Tran <quinn.tran@qlogic.com>
Cc: Saurav Kashyap <saurav.kashyap@qlogic.com>
Cc: Giridhar Malavali <giridhar.malavali@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>