kmod/core: remove unused old_func_addr field

Not needed, we can always add it again later if needed (for sanity
checks, maybe)
kmod/core: print module load/unload messages

Print the loading/unloading messages after they have successfully
completed.  Using the KERN_NOTICE log level which corresponds to a
"normal but significant condition."
kmod/core: remove obsolete kpatch_rela struct
kmod/core: move kpatch_remove_patch

Just move kpatch_remove_patch to a more logical location next to
kpatch_apply_patch.  No functional changes.
kmod/core: rename some variables

For the sake of consistency and readability, rename some variables.
Also change func->old_addr_end to func->old_size.