nvme-pci: split nvme_alloc_admin_tags

Split nvme_alloc_admin_tags into a helper to actually allocate the
tag set, and one that just restarts the admin queue.  Add a local
variable for the tag_set to clean up the code a bit.

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>