bpf: use skb_pkt_type_ok helper in bpf_skb_change_type

Since we have a skb_pkt_type_ok() helper for checking the type before
mangling, make use of it instead of open coding. Follow-up to commit
8b10cab6 ("net: simplify and make pkt_type_ok() available for other
users") that came in after d2485c42

 ("bpf: add bpf_skb_change_type
helper").

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>