neighbor: NTF_PROXY is a valid ndm_flag for a dump request

When dumping proxy entries the dump request has NTF_PROXY set in
ndm_flags. strict mode checking needs to be updated to allow this
flag.

Fixes: 51183d23 ("net/neighbor: Update neigh_dump_info for strict data checking")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>