btrfs: avoid long stall when dropping a non-shared large subvolume

btrfs: avoid long stall when dropping a non-shared large subvolume

Commit 011b46c30476 ("btrfs: skip subtree scan if it's too high to avoid
low stall in btrfs_commit_transaction()") introduced a mechanism to skip
large subtree during snapshot dropping.

But even for a subvolume without any shared tree blocks, we can still
queue quite a lot of qgroup records into one transaction, and cause a
long qgroup related stall.

So also add a check against the subvolume root level, to determine if we
need to mark qgroup inconsistent.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>