nvme: factor out a nvme_ns_remove_by_nsid helper

Factor out a piece of deeply indented and logicaly separate code
from nvme_scan_ns_list into a new helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>