nvme-fabrics: only reserve a single tag

Fabrics drivers currently reserve two tags on the admin queue.  But
given that the connect command is only run on a freshly created queue
or after all commands have been force aborted we only need to reserve
a single tag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Daniel Wagner <dwagner@suse.de>