Widen the vm_page aflags field to 16 bits.

We are now out of aflags bits, whereas the "flags" field only makes use
of five of its sixteen bits, so narrow "flags" to eight bits. I have no
intention of adding a new aflag in the near future, but would like to
combine the aflags, queue and act_count fields into a single atomically
updated word. This will allow vm_page_pqstate_cmpset() to become much
simpler and is a step towards eliminating the use of the page lock array
in updating per-page queue state.

The change modifies the layout of struct vm_page, so bump
__FreeBSD_version.

Reviewed by: alc, dougm, jeff, kib
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22397