Drop the extra argument from swp_pager_meta_ctl and have it do lookup
only. Rename it swp_pager_meta_lookup. Stop checking for obj->type

swap there and assert it instead. Make the caller responsible for

the obj->type check.

Move the meta_ctl 'pop' functionality to swap_pager_unswapped, the
only place that uses it, and assume obj->type == swap there too.

Assisted by: ota_j.email.ne.jp
Reviewed by: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D22437