Elide the vm_reserv_free_page() call when PG_PCPU_CACHE is set.

Pages with PG_PCPU_CACHE set cannot have been allocated from a
reservation, so as an optimization, skip the call to
vm_reserv_free_page() in this case. Otherwise, the access of
the corresponding reservation structure often results in a cache
miss.

Reviewed by: alc, kib
Discussed with: jeff
MFC after: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20859