debugfs: Fix lockdown check for mmap_prepare

Commit 651fdda840 ("relay: update relay to use mmap_prepare")
changed the `mmap` file operation to `mmap_prepare` for relayfs, but
the lockdown check in debugfs was not updated accordingly.

This prevents debugfs from being locked down when the kernel is in
integrity mode if a file uses `mmap_prepare` but not `mmap`.

Since the conversion to `mmap_prepare` across the kernel is not yet
complete, update the lockdown check to look for both `mmap` and
`mmap_prepare` to ensure comprehensive coverage.

Fixes: 651fdda840 ("relay: update relay to use mmap_prepare")
Signed-off-by: Chun-Yi Lee <jlee@suse.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: driver-core@lists.linux.dev
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
Tested-by: Disha Goel <disgoel@linux.ibm.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Link: https://patch.msgid.link/20260615104750.1000-1-jlee@suse.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

fs/debugfs/file.c

@@ -273,7 +273,8 @@ static int debugfs_locked_down(struct inode *inode,
 	    (!real_fops ||
 	     (!real_fops->unlocked_ioctl &&
 	      !real_fops->compat_ioctl &&
-	      !real_fops->mmap)))
+	      !real_fops->mmap &&
+	      !real_fops->mmap_prepare)))
 		return 0;
 
 	if (security_locked_down(LOCKDOWN_DEBUGFS))