rust_binderfs: fix ida_alloc_max() upper bound

The 'max' argument of ida_alloc_max() takes the maximum valid ID and not
the "count". Using an ID of BINDERFS_MAX_MINOR (1 << 20) for dev->minor
would exceed the limits of minor numbers (20-bits). Fix this off-by-one
error by subtracting 1 from the 'max'.

Cc: [e-mail]
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Signed-off-by: Carlos Llamas <[e-mail]>
Reviewed-by: Alice Ryhl <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/android/binder/rust_binderfs.c

···
 	mutex_lock(&binderfs_minors_mutex);
 	if (++info->device_count <= info->mount_opts.max)
 		minor = ida_alloc_max(&binderfs_minors,
-				      use_reserve ? BINDERFS_MAX_MINOR :
-						    BINDERFS_MAX_MINOR_CAPPED,
+				      use_reserve ? BINDERFS_MAX_MINOR - 1 :
+						    BINDERFS_MAX_MINOR_CAPPED - 1,
 				      GFP_KERNEL);
 	else
 		minor = -ENOSPC;
···
 	/* Reserve a new minor number for the new device. */
 	mutex_lock(&binderfs_minors_mutex);
 	minor = ida_alloc_max(&binderfs_minors,
-			      use_reserve ? BINDERFS_MAX_MINOR :
-					    BINDERFS_MAX_MINOR_CAPPED,
+			      use_reserve ? BINDERFS_MAX_MINOR - 1 :
+					    BINDERFS_MAX_MINOR_CAPPED - 1,
 			      GFP_KERNEL);
 	mutex_unlock(&binderfs_minors_mutex);
 	if (minor < 0) {