nvme: Count number of alginment splits

[imp]

Summary

When possible, we split up I/Os to NVMe drives that advertise a
preferred alignment. Add a counter for this.

Sponsored by: Netflix

[gallatin]

sys/dev/nvme/nvme_ns.c

431

You should be using the counter api, not atomics, for something like this.

sys/dev/nvme/nvme_private.h

323

Or if you really prefer an atomic, at least put it on a cacheline by itself.

[imp]

sys/dev/nvme/nvme_ns.c

431

Yea. This counter should be totally cold... but if it's not, we don't want it to mess thing up...

[imp]

Move to counters for this.

[chuck]

sys/dev/nvme/nvme_sysctl.c

426

The first couple of times I read the description, it sounded like this variable tracked how many splits were in an I/O. Would "Number of I/Os split to get better alignment" convey the right meaning of this sysctl or am I still confused?

Regardless of how this gets worded, maybe drop "namespace" as the counter is per controller.

[mav]

sys/dev/nvme/nvme_ctrlr.c

1445

I have feeling there should be counter_u64_free() somewhere.

[imp]

Update per review comments: description and cleanup.