Add OBJ_PG_DTOR flag to VM object.

Setting this flag allows us to skip pages removal from VM object queue
during object termination and to leave that for cdev_pg_dtor function.

Move pages removal code to separate function vm_object_terminate_pages()
as comments does not survive indentation.

This will be required for Intel SGX support where we will have to remove
pages from VM object manually.

Reviewed by: kib, alc
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D11688