refactor rela_equal

Make it easier to read and reduce the indent levels
use kpatch_mangled_strcmp in rela_equal

Use kpatch_mangled_strcmp() to compare the prefixes of special static
locals.
change special_static_prefix to is_special_static

We no longer need to return the prefix, so change it to a boolean
function.
refactor is_special_static

Make is_special_static()'s implementation more generic to make it easier
to add special static variables in the future
make _rs a special static local

The _rs variable is used for printk ratelimiting, similar to __warned,
which makes it a logical candidate to be "special": don't correlate it,
yet don't mark a function as changed just because it references it.
fix review comments

- rela sections don't have secsyms
- add some comments
Merge pull request #453 from jpoimboe/rs-special-static

make _rs a "special" static local variable