nvmet: handle keep-alive timer when kato is modified by a set features cmd

A user may modify the kato by a set features cmd.  To properly deal
with races or a kato value of 0 (no keep alive enabled) change
nvmet_set_feat_kato to first disable the timer, then set the value
and then re-enable the timer.

Signed-off-by: Amit Engel <amit.engel@dell.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>