xfs: simplify device handling

We removed all codepaths where s_umount is taken beneath open_mutex and
bd_holder_lock so don't make things more complicated than they need to
be and hold s_umount over block device opening.

Link: https://lore.kernel.org/r/20231024-vfs-super-rework-v1-2-37a8aa697148@kernel.org


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