Clean up Ksplice's locking of the update.

Clean up Ksplice's locking of the update.

Updates, and in particular update->stage, was previously not protected
by any lock.  Since the ability of Ksplice to correctly operate
requires that module_mutex be held during the entire update
application process, use module_mutex as the lock protecting
update->stage.  The actual effect in terms of fraction of time holding
module_mutex is negligible, but now we don't have to worry about races
where e.g. two threads write applied to the stage file simultaneously.

Signed-off-by: Tim Abbott <tabbott@mit.edu>