tmpfs_vnops: move swap work to swap_pager

Two functions in tmpfs_vnops.c use an interface provided by
swap_pager.c. Move most of the implementation of those functions to
swap_pager.c so that they can be implemented more effectively, with
access to implementation details of the swap pager.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D47212