net: filter: fix SKF_AD_PKTTYPE extension on big-endian

BPF classic->internal converter broke SKF_AD_PKTTYPE extension, since
pkt_type_offset() was failing to find skb->pkt_type field which is defined as:
__u8 pkt_type:3,
     fclone:2,
     ipvs_property:1,
     peeked:1,
     nf_trace:1;

Fix it by searching for 3 most significant bits and shift them by 5 at run-time

Fixes: bd4cf0ed ("net: filter: rework/optimize internal BPF interpreter's instruction set")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Tested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>