btrfs-progs: convert: Fix bugs in backup superblock migration

btrfs-progs: convert: Fix bugs in backup superblock migration

New convert has several bugs with backup superblock migration

1) Backup superblocks are not migrated due to incorrect condition
   Two wrong checks cause backup superblocks not to be migrated at all

2) Converted ext* image doesn't keep hole for backup superblocks
   Since we are creating file extents according to tmp_used, which has
   wiped out backup superblock ranges.
   In that case, later superblock migration will fail, since migration
   will insert file extent range into ext* image.

Fix above bugs will make convert on ext2 image filled about 100M data
successful.

Reported-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>