bpf: Simplify ptr_min_max_vals adjustment

An upcoming commit will add another two pointer types that need very
similar behaviour, so generalise this function now.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>