vm_object: Modify vm_object_allocate_anon() to return OBJT_SWAP objects

With this change, OBJT_DEFAULT objects are no longer allocated.
Instead, anonymous objects are always of type OBJT_SWAP and always have
OBJ_SWAP set.

Modify the page fault handler to check the swap block radix tree in
places where it checked for objects of type OBJT_DEFAULT. In
particular, there's no need to invoke getpages for an OBJT_SWAP object
with no swap blocks assigned.

Reviewed by: alc, kib
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35785