selftests: liveupdate: add test for double preservation

selftests: liveupdate: add test for double preservation

Verify that a file can only be preserved once across all active sessions.
Attempting to preserve it a second time, whether in the same or a
different session, should fail with EBUSY.

Link: https://lore.kernel.org/20260326163943.574070-4-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>