nvme: release namespace head reference on error

If a namespace identification does not match the subsystem's head for
that NSID, release the reference that was taken when the matching head
was initially found.

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