demand_paging: eviction/nru: fix pathological behavior

Currently, the NRU algorithm always picks the lowest (and very often the
same) clean unaccessed page over and over when e.g. doing large anonymous
memory mappings. Spread the eviction selection more uniformly by by
starting the search right after the last victim instead of always
restarting from 0.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>