kpatch-patch-hook: fix review comments

sysfs will be initialized on patch module init regardless of whether
or not the module is loaded. func_old_addr_show() will read from func->old_addr,
which is initially set to 0; it'll be eventually filled in by the core module.
consolidate variables func->old_offset and func->old_addr to just old_addr

To reduce redundancy, remove/change the old_offset fields in the
kpatch_func and kpatch_patch_func structs to just old_addr. Since old_offset is
being used as a placeholder for old_addr, might as well
consolidate it to just one variable.