super: remove pointless s_root checks

The locking guarantees that the superblock is alive and sb->s_root is
still set. Remove the pointless check.

Link: https://lore.kernel.org/r/20250329-work-freeze-v2-1-a47af37ecc3d@kernel.org


Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>