Future changes to posixshm will start tracking writeable mappings in order
to support file sealing. Tracking writeable mappings for an OBJT_DEFAULT
object is complicated as it may be swapped out and converted to an
OBJT_SWAP. One may generically add this tracking for vm_object, but this is
difficult to do without increasing memory footprint of vm_object and blowing
up memory usage by a significant amount.

On the other hand, the swap pager can be expanded to track writeable
mappings without increasing vm_object size. This change is currently in
D21456. Switch over to OBJT_SWAP in advance of the other changes to the
swap pager and posixshm.