nvmet: Call fatal_error from keep-alive timout expiration

We only need to call delete_ctrl once, so given that both
keep-alive timeout and any other fatal error can trigger it,
just make sure we only call delete_ctrl once.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>