Btrfs: fix wrong missing device counter decrease

The missing devices are accounted by its own fs device, for example
the missing devices in seed filesystem will be accounted by the fs device
of the seed filesystem, not by the new filesystem which is based on
the seed filesystem, so when we remove the missing device in the
seed filesystem, we should decrease the counter of its own fs device.
Fix it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>