Use ReadMaps() if page flags are specified

Smaps() doesn't iterate page-by-page, meaning that procrank and librank
cannot filter pages for KPF_SWAPBACKED. This change makes ProcessRecord
fall back on the slower ReadMaps() if page flags need to be inspected.

Test: Run procrank with -c/-C, compare PSS to procrank with no options.
Bug: 335586795
Change-Id: I167be0ca22849e0dc66e6ed96180b5405cc022de
1 file changed