libbpf: Fix overflow in BTF sanity checks

btf_header's str_off+str_len or type_off+type_len can overflow as they
are u32s. This will lead to bypassing the sanity checks during BTF
parsing, resulting in crashes afterwards. Fix by using 64-bit signed
integers for comparison.

Fixes: d8123624 ("libbpf: Fix BTF data layout checks and allow empty BTF")
Reported-by: Evgeny Vereshchagin <evvers@ya.ru>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211023003157.726961-1-andrii@kernel.org