selftests: rtnetlink: make kci_test_encap() return sub-test result

kci_test_encap() is actually composed by two different sub-tests,
kci_test_encap_vxlan() and kci_test_encap_fou()

Therefore we should check the test result of these two in
kci_test_encap() to let the script be aware of the pass / fail status.
Otherwise it will generate false-negative result like below:
    $ sudo ./test.sh
    PASS: policy routing
    PASS: route get
    PASS: preferred_lft addresses have expired
    PASS: promote_secondaries complete
    PASS: tc htb hierarchy
    PASS: gre tunnel endpoint
    PASS: gretap
    PASS: ip6gretap
    PASS: erspan
    PASS: ip6erspan
    PASS: bridge setup
    PASS: ipv6 addrlabel
    PASS: set ifalias 5b193daf-0a08-46d7-af2c-e7aadd422ded for test-dummy0
    PASS: vrf
    PASS: vxlan
    FAIL: can't add fou port 7777, skipping test
    PASS: macsec
    PASS: bridge fdb get
    PASS: neigh get
    $ echo $?
    0

Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>