cachefiles: Fix file burial to take lock when unsetting S_KERNEL_FILE

cachefiles: Fix file burial to take lock when unsetting S_KERNEL_FILE

Fix cachefiles_bury_object() to lock the inode of the file being buried
whilst it unsets the S_KERNEL_FILE flag.

Fixes: 07a90e97400c ("cachefiles: Implement culling daemon commands")
Closes: https://sashiko.dev/#/patchset/20260616100821.2062304-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260625140640.3116900-5-dhowells@redhat.com
cc: Paulo Alcantara <pc@manguebit.org>
cc: NeilBrown <neil@brown.name>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>