Implement atomic state updates using the new vm_page_astate_t structure.

Introduce primitives vm_page_astate_load() and vm_page_astate_fcmpset()
to operate on the 32-bit per-page atomic state. Modify
vm_page_pqstate_fcmpset() to use them. No functional change intended.

Introduce PGA_QUEUE_OP_MASK, a subset of PGA_QUEUE_STATE_MASK that only
includes queue operation flags. This will be used in subsequent
patches.

Reviewed by: alc, jeff, kib
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D22753