btrfs: add get_tree callback for new mount API

This is the actual mounting callback for the new mount API.  Implement
this using our current fill super as a guideline, making the appropriate
adjustments for the new mount API.

Our old mount operation had two fs_types, one to handle the actual
opening, and the one that we called to handle the actual opening and
then did the subvol lookup for returning the actual root dentry.  This
is mirrored here, but simply with different behaviors for ->get_tree.
We use the existence of ->s_fs_info to tell which part we're in.  The
initial call allocates the fs_info, then call mount_fc() with a
duplicated fc to do the actual open_ctree part.  Then we take that
vfsmount and use it to look up our subvolume that we're mounting and
return that as our s_root.  This idea was taken from Christians attempt
to convert us to the new mount API [1].

In btrfs_get_tree_super() the mount device is scanned and opened in one
go under uuid_mutex we expect that all related devices have been already
scanned, either by mount or from the outside. A device forget can be
called on some of the devices as the whole context is not protected but
it's an unlikely event, though it's a minor behaviour change.

References: 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>
[ add note about device scanning ]
Signed-off-by: David Sterba <dsterba@suse.com>