module: simplify load_module.

Linus' recent catch of stack overflow in load_module lead me to look
at the code.  A couple of helpers to get a section address and get
objects from a section can help clean things up a little.

(And in case you're wondering, the stack size also dropped from 328 to
284 bytes).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/module.h

@@ -277,7 +277,7 @@ struct module
 
 	/* Exception table */
 	unsigned int num_exentries;
-	const struct exception_table_entry *extable;
+	struct exception_table_entry *extable;
 
 	/* Startup function. */
 	int (*init)(void);