selftests/ring-buffer: Add test for out-of-bound pgoff mapping

Extend the ring-buffer mapping test coverage by checking an out-of-bound
pgoff which has proven to be problematic in the past.

Link: https://lore.kernel.org/r/20241218170318.2814991-1-vdonnefort@google.com


Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>