staging: gasket: sysfs: fix potential null dereference

Add handling of possible allocation failure.

Reported by smatch:

  drivers/staging/gasket/gasket_sysfs.c:105 put_mapping() error: potential null dereference 'files_to_remove'. (kcalloc returns null)
Signed-off-by: Ivan Bornyakov <brnkv.i1@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gasket/gasket_sysfs.c

@@ -101,6 +101,11 @@ static void put_mapping(struct gasket_sysfs_mapping *mapping)
 		files_to_remove = kcalloc(num_files_to_remove,
 					  sizeof(*files_to_remove),
 					  GFP_KERNEL);
+		if (!files_to_remove) {
+			mutex_unlock(&mapping->mutex);
+			return;
+		}
+
 		for (i = 0; i < num_files_to_remove; i++)
 			files_to_remove[i] = mapping->attributes[i].attr;