kselftest: rtnetlink: fix ip route command typo

The blamed commit below introduced a typo causing 'gretap' test-case
failures:

./rtnetlink.sh  -t kci_test_gretap -v
COMMAND: ip link add name test-dummy0 type dummy
COMMAND: ip link set test-dummy0 up
COMMAND: ip netns add testns
COMMAND: ip link help gretap 2>&1 | grep -q '^Usage:'
COMMAND: ip -netns testns link add dev gretap00 type gretap seq key 102 local 172.16.1.100 remote 172.16.1.200
COMMAND: ip -netns testns addr add dev gretap00 10.1.1.100/24
COMMAND: ip -netns testns link set dev gretap00 ups
    Error: either "dev" is duplicate, or "ups" is a garbage.
COMMAND: ip -netns testns link del gretap00
COMMAND: ip -netns testns link add dev gretap00 type gretap external
COMMAND: ip -netns testns link del gretap00
FAIL: gretap

Fix it by using the correct keyword.

Fixes: 9c2a19f7

 ("kselftest: rtnetlink.sh: add verbose flag")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>