swap_pager: iter in haspage, lookup, getpages

Use pctrie iterators for swblk traversal in more swap_pager
functions: swap_pager_haspage, swp_pager_meta_lookup, and
swap_pager_getpages.

Reported by: markj
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D47232