Lock reservations with a dedicated lock in each reservation. Protect the
vmd_free_count with atomics.

This allows us to allocate and free from reservations without the free lock
except where a superpage is allocated from the physical layer, which is
roughly 1/512 of the operations on amd64.

Use the counter api to eliminate cache conention on counters.

Reviewed by: markj
Tested by: pho
Sponsored by: Netflix, Dell/EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14707