libfc: Fix a race in fc_exch_timer_set_locked()

It is allowed to pass a zero timeout value to fc_seq_exch_abort().
Avoid that this can cause the timeout function to drop the exchange
reference before it has been increased by fc_exch_timer_set_locked().
This patch fixes a crash when running FCoE target code with poisoning
enabled in the memory allocator.

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>