net: rtnetlink.h -- only include linux/netdevice.h when used by the kernel

The commit below added a new helper dev_ingress_queue to cleanly obtain the
ingress queue pointer. This necessitated including 'linux/netdevice.h':

commit 24824a09e35402b8d58dcc5be803a5ad3937bdba
Author: Eric Dumazet <[e-mail]>
Date: Sat Oct 2 06:11:55 2010 +0000

net: dynamic ingress_queue allocation

However this include triggers issues for applications in userspace
which use the rtnetlink interfaces. Commonly this requires they include
'net/if.h' and 'linux/rtnetlink.h' leading to a compiler error as below:

In file included from /usr/include/linux/netdevice.h:28:0,
from /usr/include/linux/rtnetlink.h:9,
from t.c:2:
/usr/include/linux/if.h:135:8: error: redefinition of ‘struct ifmap’
/usr/include/net/if.h:112:8: note: originally defined here
/usr/include/linux/if.h:169:8: error: redefinition of ‘struct ifreq’
/usr/include/net/if.h:127:8: note: originally defined here
/usr/include/linux/if.h:218:8: error: redefinition of ‘struct ifconf’
/usr/include/net/if.h:177:8: note: originally defined here

The new helper is only defined for the kernel and protected by __KERNEL__
therefore we can simply pull the include down into the same protected
section.

Signed-off-by: Andy Whitcroft <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/rtnetlink.h

···
 #include <linux/if_link.h>
 #include <linux/if_addr.h>
 #include <linux/neighbour.h>
-#include <linux/netdevice.h>
 
 /* rtnetlink families. Values up to 127 are reserved for real address
  * families, values above 128 may be used arbitrarily.
···
 #ifdef __KERNEL__
 
 #include <linux/mutex.h>
+#include <linux/netdevice.h>
 
 static __inline__ int rtattr_strcmp(const struct rtattr *rta, const char *str)
 {