rtnetlink: Add rtnexthop offload flag to compare mask

[ Upstream commit 85dda4e5 ]

The offload flag is a status flag and should not be used by
FIB semantics for comparison.

Fixes: 37ed9493 ("rtnetlink: add RTNH_F_EXTERNAL flag for fib offload")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>