Btrfs: make sure to update total_bitmaps when freeing cache V3

A user reported this bug again where we have more bitmaps than we are supposed
to.  This is because we failed to load the free space cache, but don't update
the ctl->total_bitmaps counter when we remove entries from the tree.  This patch
fixes this problem and we should be good to go again.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>