vfs: lockless lookup

Provides full scalability as long as all visited filesystems support the
lookup and terminal vnodes are different.

Inner workings are explained in the comment above cache_fplookup.

Capabilities and fd-relative lookups are not supported and will result in
immediate fallback to regular code.

Symlinks, ".." in the path, mount points without support for lockless lookup
and mismatched counters will result in an attempt to get a reference to the
directory vnode and continue in regular lookup. If this fails, the entire
operation is aborted and regular lookup starts from scratch. However, care is
taken that data is not copied again from userspace.

Sample benchmark:
incremental -j 104 bzImage on tmpfs:
before: 142.96s user 1025.63s system 4924% cpu 23.731 total
after: 147.36s user 313.40s system 3216% cpu 14.326 total

Sample microbenchmark: access calls to separate files in /tmpfs, 104 workers, ops/s:
before: 2165816
after: 151216530

Reviewed by: kib
Tested by: pho (in a patchset)
Differential Revision: https://reviews.freebsd.org/D25578