nvmet: avoid queuing keep-alive timer if it is disabled

Issue following command:
nvme set-feature -f 0xf -v 0 /dev/nvme1n1 # disable keep-alive timer
nvme admin-passthru -o 0x18 /dev/nvme1n1  # send keep-alive command
will make keep-alive timer fired and thus delete the controller like
below:

[247459.907635] nvmet: ctrl 1 keep-alive timer (0 seconds) expired!
[247459.930294] nvmet: ctrl 1 fatal error occurred!

Avoid this by not queuing delayed keep-alive if it is disabled when
keep-alive command is received from the admin queue.

Signed-off-by: Hou Pu <houpu.main@gmail.com>
Tested-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>