memfd: implement get_id for memfd_luo

memfd: implement get_id for memfd_luo

Memfds are identified by their underlying inode.  Implement get_id for
memfd_luo to return the inode pointer.  This prevents the same memfd from
being managed twice by LUO if the same inode is pointed by multiple file
objects.

Link: https://lore.kernel.org/20260326163943.574070-3-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Cc: David Matlack <dmatlack@google.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Samiullah Khawaja <skhawaja@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>