super: make locking naming consistent

Make the naming consistent with the earlier introduced
super_lock_{read,write}() helpers.

Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230818-vfs-super-fixes-v3-v3-2-9f0b1876e46b@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>