vm_object: Modify various drivers to allocate OBJT_SWAP objects

This is in preparation for removal of OBJT_DEFAULT. In particular, it
is now cheap to check whether an OBJT_SWAP object has any swap blocks
allocated, so the benefit of having a separate OBJT_DEFAULT type is
quite marginal, and the OBJT_DEFAULT->SWAP transition is a source of
bugs.

Reviewed by: alc, hselasky, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35779