Btrfs: Clear space_info full when adding new devices

Btrfs: Clear space_info full when adding new devices

The full flag on the space info structs tells the allocator not to try
and allocate more chunks because the devices in the FS are fully allocated.

When more devices are added, we need to clear the full flag so the allocator
knows it has more space available.

Signed-off-by: Chris Mason <chris.mason@oracle.com>