selftests/arm64: Log errors in verify_mte_pointer_validity()

When we detect a problem in verify_mte_pointer_validity() while checking
tags we don't log what the problem was which makes debugging harder. Add
some diagnostics.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220510164520.768783-2-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>