btrfs: run clean_dirty_subvols if we fail to start a trans

If we do merge_reloc_roots() we could insert a few roots onto the dirty
subvol roots list, where we hold a ref on them.  If we fail to start the
transaction we need to run clean_dirty_subvols() in order to cleanup the
refs.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>