Btrfs: add btrfs_compare_trees function

This function is used to find the differences between
two trees. The tree compare skips whole subtrees if it
detects shared tree blocks and thus is pretty fast.

Signed-off-by: Alexander Block <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Reviewed-by: Arne Jansen <[email protected]>
Reviewed-by: Jan Schmidt <[email protected]>
Reviewed-by: Alex Lyakas <[email protected]>

Author: Alexander Block