KCOV: suppress parasitic coverage

[dvyukov]

Some common kernel helper functions, especially the ones with loop, or called inside of loops, can produce lots of uninteresting coverage that overflows KCOV buffer. We already don't instrument some of such helpers, but we need to support more.

These can be found by running programs with beefy coverage (esp the ones that overflow KCOV buffer) with syz-execprog -coverfile, and then passing the resulting traces via addr2line.

Some helpers that I found:

• is_bpf_text_address called during stack unwinding
• arch_static_branch is called a lot from somewhere
• string_nocheck has a loop
• common containers (lib/xarray.c, lib/maple_tree.c) (are they interesting for us?)

[jiangenj]

Add discussion link here https://groups.google.com/g/syzkaller/c/rSPSMttoNWw
and print pc count PR #5617 since syz-execprog, then addr2line takes much time.