unix: Use a dedicated mtx pool for vnode name locks

[jhb]

Summary

mtxpool_sleep should be used as a leaf lock since it is a general
purpose pool shared across many consumers. Holding a pool lock while
acquiring other locks (e.g. the socket buffer lock in soreserve()) can
trigger LOR warnings for unrelated code.

Sponsored by: Chelsio Communications

[jhb]

The LORs I was seeing before were mitigated by rGaec2ae8b57fc638fa3ba9fac9d2067f2049ab613. However, I do think in general that locks from the global mutex pools should only be used as leaf locks.

Also, the count of 1024 locks might be too high here. I just copied the default size of the mtxpool_sleep pool. We could use a smaller value like 128 or 64 if we don't think these will be contested very often.

[glebius]

We definitely should go with a much lower value. The pool is used only during connect/close/detach of a named socket, which is definitely not a hot path in any sane workload.

[jhb]

Ok, I'll drop it to 32 for commit then.