vnode_pager: clean up undirty_pages()

The first (second) loop of vnode_pager_undirty_pages() includes an
if-test to test whether an iteration is the last (first). Move those
if-tests out of their loops. That allows the compiler to unroll the
loops.

Reviewed by: alc, kib
Differential Revision: https://reviews.freebsd.org/D54353