nvme: Don't create sysctl for io queues not created

When we can't set the number of I/O queues ont he admin queue, we
continue on. However, we don't create the I/O queue structures, so
having pointers (NULL) into them for sysctls makes no sense and leads to
a panic when accessed. When summing up different stats, also skip the
ioq stats when it's NULL.

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