bpf: Omit superfluous address family check in __bpf_skc_lookup

family is only set to either AF_INET or AF_INET6 based on len. In all
other cases we return early. Thus the check against AF_UNSPEC can be
omitted.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220630082618.15649-1-tklauser@distanz.ch