nvme-core: remove repeated wq flags

In nvme_core_init() nvme_wq, nvme_reset_wq, nvme_delete_wq share same
flags :- WQ_UNBOUND | WQ_MEM_RECLAIM | WQ_SYSFS.

Insated of repeating these flags in each call use the common variable.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>