nvme-core: do not pass empty queue_limits to blk_mq_alloc_queue()

nvme-core: do not pass empty queue_limits to blk_mq_alloc_queue()

In nvme_alloc_admin_tag_set(), an empty queue_limits struct is
currently allocated on the stack and passed by reference to
blk_mq_alloc_queue().

This is redundant because blk_mq_alloc_queue() already handles
a NULL limits pointer by internally substituting it with a default
empty queue_limits struct.
Remove the unnecessary local variable and pass a NULL value.

Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>