Btrfs: Add shared reference cache

Btrfs has a cache of reference counts in leaves, allowing it to
avoid reading tree leaves while deleting snapshots.  To reduce
contention with multiple subvolumes, this cache is private to each
subvolume.

This patch adds shared reference cache support. The new space
balancing code plays with multiple subvols at the same time, So
the old per-subvol reference cache is not well suited.

Signed-off-by: Chris Mason <chris.mason@oracle.com>