generic/756: test name_to_handle_at(AT_HANDLE_MNT_ID_UNIQUE) explicitly

generic/756: test name_to_handle_at(AT_HANDLE_MNT_ID_UNIQUE) explicitly

In order to make sure we are actually testing AT_HANDLE_MNT_ID_UNIQUE,
add a test (based on generic/426) which runs the open_by_handle in a
mode where it will error out if there is a problem with getting mount
IDs. The test is skipped if the kernel doesn't support the necessary
features.

Suggested-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>