ipv4/fib_frontend: Allow RTM_F_CLONED flag to be used for filtering

This functionally reverts the check introduced by commit
e8ba330a ("rtnetlink: Update fib dumps for strict data checking")
as modified by commit e4e92fb1

 ("net/ipv4: Bail early if user only
wants prefix entries").

As we are preparing to fix listing of IPv4 cached routes, we need to
give userspace a way to request them.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>