selftests: fib rule: Log test description

All callers of fib_rule6_test_match_n_redirect() and
fib_rule4_test_match_n_redirect() pass a third argument containing a
description of the test being run. Instead of ignoring this argument,
let's use it for logging instead of printing a truncated version of the
command.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>