net/af_iucv: remove GFP_DMA restriction for HiperTransport

af_iucv sockets over z/VM IUCV require that their skbs are allocated
in DMA memory. This restriction doesn't apply to connections over
HiperSockets. So only set this limit for z/VM IUCV sockets, thereby
increasing the likelihood that the large (and linear!) allocations for
HiperTransport messages succeed.

Fixes: 3881ac44

 ("af_iucv: add HiperSockets transport")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>