There's no need to drop s_umount anymore now that we removed all sources
where s_umount is taken beneath open_mutex or bd_holder_lock.

Link: https://lore.kernel.org/r/20231024-vfs-super-rework-v1-1-37a8aa697148@kernel.orgReviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>