ibmvnic: Unmap longer term buffer before free

Make sure we unregister long term buffers from the adapter
prior to DMA unmapping it and freeing the buffer. Failure
to do so could result in a DMA to a now invalid address.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>