Btrfs-progs: init free space ctl with proper unit

Btrfs-progs: init free space ctl with proper unit

btrfsck was blowing up when checking the free space cache when we ran xfstests
with -l 64k.  That is because I was init'ing the free space ctl to whatever the
leafsize was, which isn't right for data block groups.  With this patch btrfsck
no longer complains.  This also fixes a tiny little typo in free-space-cache.c I
noticed while figuring this problem out.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>