btrfs: test btrfs send after complex directory hierarchy changes

btrfs: test btrfs send after complex directory hierarchy changes

Test a very complex scenario for a btrfs incremental send operation where a
large directory hierarchy had many subtrees moved between parent directories,
preserving the names of some directories and inverting the parent-child
relationship between some directories (a child in the parent snapshot became
a parent, in the send snapshot, of the directory that is its parent in the
parent snapshot).

This test made the incremental send fail with -ENOMEM because it entered an
infinite loop when building path strings that are used as operands of the
rename operations issued in the send stream.
This issue was fixed by the following linux kernel btrfs patch:

   Btrfs: incremental send, don't delay directory renames unnecessarily

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Dave Chinner <david@fromorbit.com>