NTB: Out of free receive entries issue

If the NTB client driver enqueues the maximum number of rx buffers, it
will not be able to re-enqueue another in its callback handler due to a
lack of free entries.  This can be avoided by adding the current entry
to the free queue prior to calling the client callback handler.  With
this change, ntb_netdev will no longer encounter a rx error on its first
packet.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>