nvme-fc: convert assoc_active flag to bit op

Convert the assoc_active boolean flag to a bitop on the flags field.
The bit ops will provide atomicity.

To make this change, the flags field was converted to a long type,
which also affects the FCCTRL_TERMIO flag.  Both FCCTRL_TERMIO and
now ASSOC_ACTIVE flags are set/cleared by bit operations.

Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>