kmod/core: remove ftrace hacks

These hacks were from a previous implementation and are no longer
needed.
kmod/core: remove comments about preemption

Preemption shouldn't cause a problem with determining activeness safety.
Even if a thread is preempted, it'll be on the backtrace.

We may need to disable preemption when reading the kpatch_funcs array,
but I'm removing that comment for now because the kpatch_funcs array
will soon be replaced by a much better data structure, and we'll deal
with proper synchronization then.
kmod/core: remove sync_core comment

No need to call sync_core() since we aren't directly modifying any code.
kmod/core: handle kmalloc errors and fix leak
kmod/core: remove unused old_func_addr field

Not needed, we can always add it again later if needed (for sanity
checks, maybe)