nvme-pci: fix a NULL pointer dereference in nvme_alloc_admin_tags

In the Linux kernel, the following vulnerability has been resolved:

nvme-pci: fix a NULL pointer dereference in nvme_alloc_admin_tags

In nvme_alloc_admin_tags, the admin_q can be set to an error (typically

-ENOMEM) if the blk_mq_init_queue call fails to set up the queue, which

is checked immediately after the call. However, when we return the error

message up the stack, to nvme_reset_work the error takes us to

nvme_remove_dead_ctrl()

nvme_dev_disable()

nvme_suspend_queue(&dev->queues[0]).

Here, we only check that the admin_q is non-NULL, rather than not

an error or NULL, and begin quiescing a queue that never existed, leading

to bad / NULL pointer dereference.