selftests: rtnetlink: correct the final return value for the test

The return value "ret" will be reset to 0 from the beginning of each
sub-test in rtnetlink.sh, therefore this test will always pass if the
last sub-test has passed:
    $ sudo ./rtnetlink.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 a39ee707-e36b-41d3-802f-63179ed4d580 for test-dummy0
    PASS: vrf
    PASS: vxlan
    FAIL: can't add fou port 7777, skipping test
    PASS: macsec
    PASS: ipsec
    3,7c3,7
    < sa[0]    spi=0x00000009 proto=0x32 salt=0x64636261 crypt=1
    < sa[0]    key=0x31323334 35363738 39303132 33343536
    < sa[1] rx ipaddr=0x00000000 00000000 00000000 c0a87b03
    < sa[1]    spi=0x00000009 proto=0x32 salt=0x64636261 crypt=1
    < sa[1]    key=0x31323334 35363738 39303132 33343536
    ---
    > sa[0]    spi=0x00000009 proto=0x32 salt=0x61626364 crypt=1
    > sa[0]    key=0x34333231 38373635 32313039 36353433
    > sa[1] rx ipaddr=0x00000000 00000000 00000000 037ba8c0
    > sa[1]    spi=0x00000009 proto=0x32 salt=0x61626364 crypt=1
    > sa[1]    key=0x34333231 38373635 32313039 36353433
    FAIL: ipsec_offload incorrect driver data
    FAIL: ipsec_offload
    PASS: bridge fdb get
    PASS: neigh get
    $ echo $?
    0

Make "ret" become a local variable for all sub-tests.
Also, check the sub-test results in kci_test_rtnl() and return the
final result for this test.

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