btrfs-progs: fix backref after init-csum-tree

btrfs-progs: fix backref after init-csum-tree

btrfsck reports backref error after running init-csum-tree

btrfsck --init-csum-tree /dev/sdc
btrfsck /dev/sdc
::
ref mismatch on [29474816 16384] extent item 1, found 0
Backref 29474816 root 7 not referenced back 0x1101d30
Incorrect global backref count on 29474816 found 1 wanted 0
backpointer mismatch on [29474816 16384]
owner ref check failed [29474816 16384]
Errors found in extent allocation tree or chunk allocation
::

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>