kmod/core: module old_addr fix

When patching a module, I ran into a "can't set ftrace filter at
address" error.  The root cause was due to the fact that
mod->module_core + old_offset is apparently not a reliable way to
determine the function's address.

Instead, just get the address from kallsyms like we do for module
dynrelas.
Merge pull request #317 from jpoimboe/module-old-addr-fix

kmod/core: module old_addr fix