const_structs.checkpatch: add xattr_handler

Now that the vfs can handle "const struct xattr_handler" make sure that
new usages of the struct already enter the tree as const.

Link: https://lore.kernel.org/lkml/20230930050033.41174-1-wedsonaf@gmail.com/


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20231012-vfs-xattr_const-v1-1-6c21e82d4d5e@weissschuh.net


Signed-off-by: Christian Brauner <brauner@kernel.org>