nvme: introduce a helper function for controller deletion

This patch does not change any functionality but makes the next patch
in this series easier to read.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>