[SCSI] fcoe: fix handling of pending queue, prevent out of order frames (v3)

In fcoe_check_wait_queue() the queue length could temporarily drop to 0,
before the last frame was successfully sent.  This resulted in out of order
data frames within a single sequence, leading to IO timeout errors.

This builds on the approach from Vasu Dev to only fix the queue management in
fcoe_check_wait_queue, where my first patch added locking to the transmit
path even when the pending queue was not in use.

This patch continues to use fcoe_pending_queue.qlen instead of introducing a
new length counter, but takes precautions to ensure it never drops to 0 before
the final frame in the queue has successfully been passed to the netdev qdisc
layer.  It also includes some cleanup of fcoe_check_wait_queue and removes the
fcoe_insert_wait_queue(_head) wrapper functions.

Signed-off-by: Chris Leech <christopher.leech@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>