net: remove mm.h inclusion from netdevice.h

Remove linux/mm.h inclusion from netdevice.h -- it's unused (I've checked manually).

To prevent mm.h inclusion via other channels also extract "enum dma_data_direction"
definition into separate header. This tiny piece is what gluing netdevice.h with mm.h
via "netdevice.h => dmaengine.h => dma-mapping.h => scatterlist.h => mm.h".
Removal of mm.h from scatterlist.h was tried and was found not feasible
on most archs, so the link was cutoff earlier.

Hope people are OK with tiny include file.

Note, that mm_types.h is still dragged in, but it is a separate story.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/dma/ste_dma40.c b/drivers/dma/ste_dma40.c
index 8f222d4..29d1add 100644
--- a/drivers/dma/ste_dma40.c
+++ b/drivers/dma/ste_dma40.c
@@ -6,6 +6,7 @@
  * License terms: GNU General Public License (GPL) version 2
  */
 
+#include <linux/dma-mapping.h>
 #include <linux/kernel.h>
 #include <linux/slab.h>
 #include <linux/dmaengine.h>