MFC r351650, r351795-r351796: writemapping accounting for posixshm

r351650:
posixshm: switch to OBJT_SWAP in advance of other changes

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.

r351795:
vm pager: writemapping accounting for OBJT_SWAP

Currently writemapping accounting is only done for vnode_pager which does
some accounting on the underlying vnode.

Extend this to allow accounting to be possible for any of the pager types.
New pageops are added to update/release writecount that need to be
implemented for any pager wishing to do said accounting, and we implement
these methods now for both vnode_pager (unchanged) and swap_pager.

The primary motivation for this is to allow other systems with OBJT_SWAP
objects to check if their objects have any write mappings and reject
operations with EBUSY if so. posixshm will be the first to do so in order to
reject adding write seals to the shmfd if any writable mappings exist.

r351796:
posixshm: start counting writeable mappings

r351650 switched posixshm to using OBJT_SWAP for shm_object

r351795 added support to the swap_pager for tracking writeable mappings

Take advantage of this and start tracking writeable mappings; fd sealing
will use this to reject a seal on writing with EBUSY if any such mapping
exist.