btrfs: replace seed device followed by unmount causes kernel WARNING

btrfs: replace seed device followed by unmount causes kernel WARNING

reproducer:
mount /dev/sdb /btrfs
btrfs dev add /dev/sdc /btrfs
btrfs rep start -B /dev/sdb /dev/sdd /btrfs
umount /btrfs

WARNING: CPU: 0 PID: 12661 at fs/btrfs/volumes.c:891 __btrfs_close_devices+0x1b0/0x200 [btrfs]()
::

__btrfs_close_devices()
::
        WARN_ON(fs_devices->open_devices);

After the seed device has been replaced the new target device
is no more a seed device. So we need to update the device
numbers in the fs_devices as pointed by the fs_info.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>