bpf: fix BTF limits

vmlinux BTF has more than 64k types.
Its string section is also at the offset larger than 64k.
Adjust both limits to make in-kernel BTF verifier successfully parse in-kernel BTF.

Fixes: 69b693f0

 ("bpf: btf: Introduce BPF Type Format (BTF)")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>