btrfs: resume balance on mount with qgroup

btrfs: resume balance on mount with qgroup

There are two regressions related to balance resume:
- Kernel NULL pointer dereference at mount time
  Introduced in v5.0
- Kernel BUG_ON() just after mount
  Introduced in v5.1

The kernel fixes are:
"btrfs: qgroup: Check if @bg is NULL to avoid NULL pointer
 dereference"
"btrfs: reloc: Also queue orphan reloc tree for cleanup to
 avoid BUG_ON()"

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>