mlxsw: spectrum_router: Create IPv4 specific entry struct

Some of the parameters stored in the FIB entry structure are specific to
IPv4 and therefore better placed in an IPv4 specific structure.

Create an IPv4 specific structure that encapsulates the common FIB entry
structure and contains IPv4 specific parameters.

In a follow-up patchset an IPv6 specific structure will be introduced.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>