module: clean up load_module a little more.

1fb9341a made our locking in
load_module more complicated: we grab the mutex once to insert the
module in the list, then again to upgrade it once it's formed.

Since the locking is self-contained, it's neater to do this in
separate functions.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>