selftests: fib_tests: fix link-local retrieval in fib6_nexthop()

selftests: fib_tests: fix link-local retrieval in fib6_nexthop()

fib6_nexthop() retrieves the link-local address for two interfaces used
in the test. However, both lldummy and llv1 are obtained from dummy0.

llv1 is expected to be retrieved from veth1, which is the interface used
later in the test. The subsequent check and error message also expect
the address to be retrieved from veth1.

Fix this by retrieving llv1 from veth1.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Link: https://patch.msgid.link/20260306180830.2329477-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>