bpf: sync bpf.h to tools/

Update bpf_sock_addr comments to indicate support for 8-byte reads
from user_ip6 and msg_src_ip6.

Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>