btrfs: handle the ro->rw transition for mounting different subvolumes

This is a special case that we've carried around since 0723a047 ("btrfs:
allow mounting btrfs subvolumes with different ro/rw options") where
we'll under the covers flip the file system to RW if you're mixing and
matching ro/rw options with different subvol mounts.  The first mount is
what the super gets setup as, so we'd handle this by remount the super
as rw under the covers to facilitate this behavior.

With the new mount API we can't really allow this, because user space
has the ability to specify the super block settings, and the mount
settings.  So if the user explicitly sets the super block as read only,
and then tried to mount a rw mount with the super block we'll reject
this.  However the old API was less descriptive and thus we allowed this
kind of behavior.

This patch preserves this behavior for the old API calls.  This is
inspired by Christians work [1], and includes his comment in
btrfs_get_tree_super() explaining the history and how it all works in
the old and new APIs.

Link: https://lore.kernel.org/all/20230626-fs-btrfs-mount-api-v1-2-045e9735a00b@kernel.org/


Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>