nvme_fc/nvmet_fc: revise Create Association descriptor length

Revises the Create Association LS for the amount of pad expected in 1.16.

Add defines for the minimum lengths that a target can accept (e.g. variable
pad lengths)

Signed-off-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>