nvmet: make kato and AEN processing for use by other controllers

Make common process of get/set features available to other controllers by
making simple functions static inline and others not static and prototypes
in nvmet.h file

Also remove static from nvmet_execute_async_event and add prototype to
nvmet.h to allow used by other controllers

Signed-off-by: Jay Sternberg <jay.e.sternberg@intel.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>