nvme: fine-tune sending of first keep-alive

Keep-alive commands are sent half-way through the kato period.
This normally works well but fails when the keep-alive system is
started when we are more than half way through the kato.
This can happen on larger setups or due to host delays.
With this change we now time the initial keep-alive command from
the controller initialisation time, rather than the keep-alive
mechanism activation time.

Signed-off-by: Mark O'Donovan <shiftee@posteo.net>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>