kernel/sys.c: Move overflow{uid,gid} sysctl into kernel/sys.c

Moved ctl_tables elements for overflowuid and overflowgid into in
kernel/sys.c. Create a register function that keeps them under "kernel"
and run it after core with postcore_initcall.

This is part of a greater effort to move ctl tables into their
respective subsystems which will reduce the merge conflicts in
kernel/sysctl.c.

Signed-off-by: Joel Granados <joel.granados@kernel.org>