Adapt vm_page_aflag_set(PGA_WRITEABLE) to the locking of
pmap_enter(PMAP_ENTER_NOSLEEP). The PGA_WRITEABLE flag can be set
when either the page is busied, or the owner object is locked.

Update comments, move all assertions about page state when
PGA_WRITEABLE flag is set, into new helper
vm_page_assert_pga_writeable().

Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks