selftest: net: prevent use of uninitialized variable

Fix to avoid the usage of the `ret` variable uninitialized in the
following macro expansions.

It solves the following warning:

In file included from netlink-dumps.c:21:
netlink-dumps.c: In function ‘dump_extack’:
../kselftest_harness.h:788:35: warning: ‘ret’ may be used uninitialized [-Wmaybe-uninitialized]
788 | intmax_t __exp_print = (intmax_t)__exp; \
| ^~~~~~~~~~~
../kselftest_harness.h:631:9: note: in expansion of macro ‘__EXPECT’
631 | __EXPECT(expected, #expected, seen, #seen, ==, 0)
| ^~~~~~~~
netlink-dumps.c:169:9: note: in expansion of macro ‘EXPECT_EQ’
169 | EXPECT_EQ(ret, FOUND_EXTACK);
| ^~~~~~~~~

The issue can be reproduced, building the tests, with the command:
make -C tools/testing/selftests TARGETS=net

Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Link: https://patch.msgid.link/20251023205354.28249-1-alessandro.zanni87@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>