swap_pager: examine swblks with pctrie iterators

Replace calls to pctrie lookup and remove functions, which always
begin with a search from the pctrie root, with the use of pctrie
iterators to traverse and remove items from the pctrie of swapblks
without duplicating searches. Take care to reset iterators after
reacquiring an object lock, since with the lock released they could be
invalidated.

Reviewed by: alc, markj, kib
Tested by: pho (previous versions)
Differential Revision: https://reviews.freebsd.org/D46620