MFC r322913:
Replace global swhash in swap pager with per-object trie to track swap
blocks assigned to the object pages.

MFC r322970 (by alc):
Do not call vm_pager_page_unswapped() on the fast fault path.

MFC r322971 (by alc):
Update a couple vm_object lock assertions in the swap pager.

MFC r323224:
In swp_pager_meta_build(), handle a race with other thread allocating
swapblk for our index while we dropped the object lock.

MFC r323226:
Do not leak empty swblk.