Convert bpf_update_stats to use type safe accessors

Unfortunately no way to do this in C without another macro.

Note: this doesn't result in any more code being generated, since
these were already being inlined at every invocation point, so the
code duplication is only at the source level, and even there it
is hidden behind the macro.

Test: atest libbpf_android_test libnetdbpf_test netd_integration_test netd_unit_test netdutils_test resolv_integration_test resolv_unit_test
Bug: 130746652
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Idcd903290fd36ac02d5fb6d98fe5bb809d4ec415