kselftest: Fix vdso_test_abi return status

vdso_test_abi contains a batch of tests that verify the validity of the
vDSO ABI.

When a vDSO symbol is not found the relevant test is skipped reporting
KSFT_SKIP. All the tests return values are then added in a single
variable which is checked to verify failures. This approach can have
side effects which result in reporting the wrong kselftest exit status.

Fix vdso_test_abi verifying the return code of each test separately.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>