unix: Use a dedicated mtx pool for vnode name locks

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.

Reviewed by: glebius
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D46792