selftests/bpf: Use local type for flow_offload_tuple_rhash in xdp_flowtable

selftests/bpf: Use local type for flow_offload_tuple_rhash in xdp_flowtable

Define flow_offload_tuple_rhash___local and use it in place of the
forward-declared kernel type for the bpf_xdp_flow_lookup kfunc return
type and tuplehash variable. This is consistent with how
bpf_flowtable_opts___local is already handled in the same file and
avoids relying on a forward declaration of the struct.

Fixes: eeb23b54e447 ("selftests/bpf: fix compilation failure when CONFIG_NF_FLOW_TABLE=m")
Signed-off-by: Gregory Bell <grbell@redhat.com>
Link: https://lore.kernel.org/r/20260417154122.2558890-2-grbell@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>