btrfs-progs: convert: fix unable to rollback case with removed empty block groups

btrfs-progs: convert: fix unable to rollback case with removed empty block groups

Run fstests: btrfs/012 will fail with message:
	unable to do rollback

It is because the rollback function checks sequentially each piece of space
to map to a certain block group. If some piece doesn't, rollback refuses to continue.

After kernel commit:
	commit 47ab2a6c689913db23ccae38349714edf8365e0a
	Btrfs: remove empty block groups automatically

Empty block groups are removed, so there are possible gaps:

	|--block group 1--|	|--block group 2--|
			     ^
			     |
			    gap

So the piece of space of the gap belongs to a removed empty block group,
and rollback should detect this case, and feel free to continue.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>