selftests: pmtu.sh: fix result checking

selftests: pmtu.sh: fix result checking

[ Upstream commit 63e201916b27260218e528a2f8758be47f99bbf4 ]

In the PMTU test, when all previous tests are skipped and the new test
passes, the exit code is set to 0. However, the current check mistakenly
treats this as an assignment, causing the check to pass every time.

Consequently, regardless of how many tests have failed, if the latest test
passes, the PMTU test will report a pass.

Fixes: 2a9d3716b810 ("selftests: pmtu.sh: improve the test result processing")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>