gre: use nla_get_be32() to extract flowinfo

Eliminate a sparse endianness mismatch warning, use nla_get_be32() to
extract a __be32 value instead of nla_get_u32().

Signed-off-by: Lance Richardson <lrichard@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>