netdevsim: Use dscp_t in struct nsim_fib4_rt

Use the new dscp_t type to replace the tos field of struct
nsim_fib4_rt. This ensures ECN bits are ignored and makes it compatible
with the dscp fields of struct fib_entry_notifier_info and struct
fib_rt_info.

This also allows sparse to flag potential incorrect uses of DSCP and
ECN bits.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>