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>