net/ipv4: don't use module_init in non-modular gre_offload

Recent commit 438e38fa
("gre_offload: statically build GRE offloading support") added
new module_init/module_exit calls to the gre_offload.c file.

The file is obj-y and can't be anything other than built-in.
Currently it can never be built modular, so using module_init
as an alias for __initcall can be somewhat misleading.

Fix this up now, so that we can relocate module_init from
init.h into module.h in the future.  If we don't do this, we'd
have to add module.h to obviously non-modular code, and that
would be a worse thing.  We also make the inclusion explicit.

Note that direct use of __initcall is discouraged, vs. one
of the priority categorized subgroups.  As __initcall gets
mapped onto device_initcall, our use of device_initcall
directly in this change means that the runtime impact is
zero -- it will remain at level 6 in initcall ordering.

As for the module_exit, rather than replace it with __exitcall,
we simply remove it, since it appears only UML does anything
with those, and even for UML, there is no relevant cleanup
to be done here.

Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre_offload.c

@@ -11,6 +11,7 @@
  */
 
 #include <linux/skbuff.h>
+#include <linux/init.h>
 #include <net/protocol.h>
 #include <net/gre.h>
 
@@ -283,11 +284,4 @@ static int __init gre_offload_init(void)
 {
 	return inet_add_offload(&gre_offload, IPPROTO_GRE);
 }
-
-static void __exit gre_offload_exit(void)
-{
-	inet_del_offload(&gre_offload, IPPROTO_GRE);
-}
-
-module_init(gre_offload_init);
-module_exit(gre_offload_exit);
+device_initcall(gre_offload_init);