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/include/net_user.h

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2002 Jeff Dike (jdike@karaya.com)
+ * Copyright (C) 2002 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
  * Licensed under the GPL
  */
 
@@ -18,10 +18,10 @@ struct net_user_info {
 	int (*open)(void *);
 	void (*close)(int, void *);
 	void (*remove)(void *);
-	int (*set_mtu)(int mtu, void *);
 	void (*add_address)(unsigned char *, unsigned char *, void *);
 	void (*delete_address)(unsigned char *, unsigned char *, void *);
 	int max_packet;
+	int mtu;
 };
 
 extern void ether_user_init(void *data, void *dev);