btrfs: Add a test for leaking root crash at unmount time

btrfs: Add a test for leaking root crash at unmount time

Test if canceled balance could lead to root leakage.
If the kernel has CONFIG_BTRFS_DEBUG compiled, unmount time root leakge
check would detect it, and cause NULL pointer dereference as the pages
of the leaked root are already freed.

The fix is titled "btrfs: relocation: Fix reloc root leakage and the NULL
 pointer reference caused by the leakage".

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>