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.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D21456