uml: network driver MTU cleanups

A bunch of MTU-related cleanups in the network code.

First, there is the addition of the notion of a maximally-sized packet, which
is the MTU plus headers.  This is used to size the skb that will receive a
packet.  This allows ether_adjust_skb to go away, as it was used to resize the
skb after it was allocated.

Since the skb passed into the low-level read routine is no longer resized, and
possibly reallocated, there, they (and the write routines) don't need to get
an sk_buff **.  They just need the sk_buff * now.  The callers of
ether_adjust_skb still need to do the skb_put, so that's now inlined.

The MAX_PACKET definitions in most of the drivers are gone.

The set_mtu methods were all the same and did nothing, so they can be
removed.

The ethertap driver had a typo which doubled the size of the packet rather
than adding two bytes to it.  It also wasn't defining its setup_size, causing
a zero-byte kmalloc and crash when the invalid pointer returned from kmalloc
was dereferenced.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/slip_user.c

@@ -230,11 +230,6 @@ int slip_user_write(int fd, void *buf, int len, struct slip_data *pri)
 	return slip_proto_write(fd, buf, len, &pri->slip);
 }
 
-static int slip_set_mtu(int mtu, void *data)
-{
-	return mtu;
-}
-
 static void slip_add_addr(unsigned char *addr, unsigned char *netmask,
 			  void *data)
 {
@@ -260,8 +255,8 @@ const struct net_user_info slip_user_info = {
 	.open		= slip_open,
 	.close	 	= slip_close,
 	.remove	 	= NULL,
-	.set_mtu	= slip_set_mtu,
 	.add_address	= slip_add_addr,
 	.delete_address = slip_del_addr,
-	.max_packet	= BUF_SIZE
+	.mtu		= BUF_SIZE,
+	.max_packet	= BUF_SIZE,
 };