Use the existing bounce buffer if we send a buffer under a certain size.
This saves the overhead of a TCE map/unmap.

I can't see any reason for the wmb() in the bounce buffer case, if we need
a barrier it will be before we call h_send_logical_lan but we have
nothing in the common case. Remove it.
Signed-off-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NSantiago Leon <santil@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>