btrfs: derive f_fsid with dev_t only when temp_fsid is active

Commit c2a74ed0494c ("btrfs: derive f_fsid from on-disk fsid and dev_t")
mixed dev_t into f_fsid for all single-device setups to avoid f_fsid
collisions with cloned filesystems.

However, doing this unconditionally breaks backward compatibility.
statfs(2) f_fsid changes after a kernel upgrade, and also can shift
across reboots or dev re-attaches as dev_t values change.

Fix this by only mixing dev_t when temp_fsid is active.  This means for
non-temp_fsid setups or the original mount, we use the old method of
deriving fsid based on the UUID.

So in the case of a cloned Btrfs filesystem, we won't be able to
maintain the same fsid across mount recycle if the mount order changes.

Reported-by: Dave Hansen <dave.hansen@intel.com>
Link: https://lore.kernel.org/linux-btrfs/be0c08f5-2f31-40f5-8a3b-f2f58b3e00ff@intel.com
Fixes: c2a74ed0494c ("btrfs: derive f_fsid from on-disk fsid and dev_t")
CC: stable@vger.kernel.org # 7.2
Signed-off-by: Anand Jain <asj@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>