libfc: Protect ep->esb_stat changes via ex_lock

This patch avoids that the WARN_ON(!(ep->esb_stat & ESB_ST_SEQ_INIT))
statement in fc_seq_send_locked() gets triggered sporadically when
running FCoE target code due to concurrent ep->esb_stat modifications.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>