kernel - Clean up ucred and plimit cache line locality
* Move struct plimit's p_spin and p_refcnt fields into their own
cacheline. This structure is massively shared and read often.
Doing this avoids unnecessary cache line ping-pongs.
* Only use p_spin to modify a resource limit. Do not use it to
access the resource limit.
* Integrate plimit's exclusivity flag into p_refcnt.
* Move struct ucred's cr_ref into its own cacheline. This structure
is massively shared and read often. Doing this avoids unnecessary
cache line ping-pongs.