Improve support for exporting btrfs subvolumes

If you export two subvolumes of a btrfs filesystem, they will both be
given the same uuid so lookups will be confused.
blkid cannot differentiate the two, so we must use the fsid from
statfs64 to identify the filesystem.

We cannot tell if blkid or statfs is best without knowing internal
details of the filesystem in question, so we need to encode specific
knowledge of btrfs in mountd.  This is unfortunate.

To ensure smooth handling of this and possible future changes in uuid
generation, we add infrastructure for multiple different uuids to be
recognised on old filehandles, but only the preferred on is used on
new filehandles.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>