fs: massage locking helpers

Multiple people have balked at the the fact that
super_lock{_shared,_excluse}() return booleans and even if they return
false hold s_umount. So let's change them to only hold s_umount when
true is returned and change the code accordingly.

Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-1-599c19f4faac@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>