Btrfs: fix an oops when we fail to relocate tree blocks

Btrfs: fix an oops when we fail to relocate tree blocks

During balance test, we hit an oops:
[ 2013.841551] kernel BUG at fs/btrfs/relocation.c:1174!

The problem is that if we fail to relocate tree blocks, we should
update backref cache, otherwise, some pending nodes are not updated
while snapshot check @cache->last_trans is within one transaction
and won't update it and then oops happen.

Signed-off-by: Wang Shilong <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>
Signed-off-by: Chris Mason <[email protected]>