[dfsan] Fix release_shadow_space.c (#94770)

DFSan's sscanf is incorrect
(https://github.com/llvm/llvm-project/issues/94769), which results in
erroneous matches when scraping RSS from /proc/maps. This patch works
around the issue by using strstr as a secondary check.

It also adds a loose validity check for the initial RSS measurement, to
guard against regressions in get_rss_kb().

Fixes https://github.com/llvm/llvm-project/issues/91287