Btrfs: make sure we cleanup all reloc roots if error happens

I hit an oops when merging reloc roots fails, the reason is that
new reloc roots may be added and we should make sure we cleanup
all reloc roots.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>