UPSTREAM: bpf: reserve flags in bpf_skb_net_shrink

The ENCAP flags in bpf_skb_adjust_room are ignored on decap with
bpf_skb_net_shrink. Reserve these bits for future use.

Fixes: 868d523535c2d ("bpf: add bpf_skb_adjust_room encap flags")
Change-Id: I814907392506a8f22e2ed2073da4f3cf50c74a7b
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>