kselftest: rtnetlink.sh: use grep_fail when expecting the cmd fail

kselftest: rtnetlink.sh: use grep_fail when expecting the cmd fail

run_cmd_grep_fail should be used when expecting the cmd fail, or the ret
will be set to 1, and the total test return 1 when exiting. This would cause
the result report to fail if run via run_kselftest.sh.

Before fix:
 # ./rtnetlink.sh -t kci_test_addrlft
 PASS: preferred_lft addresses have expired
 # echo $?
 1

After fix:
 # ./rtnetlink.sh -t kci_test_addrlft
 PASS: preferred_lft addresses have expired
 # echo $?
 0

Fixes: 9c2a19f71515 ("kselftest: rtnetlink.sh: add verbose flag")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231219065737.1725120-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>