Fix up possibly racy module refcounting

Module refcounting is implemented with a per-cpu counter for speed.
However there is a race when tallying the counter where a reference may
be taken by one CPU and released by another.  Reference count summation
may then see the decrement without having seen the previous increment,
leading to lower than expected count.  A module which never has its
actual reference drop below 1 may return a reference count of 0 due to
this race.

Module removal generally runs under stop_machine, which prevents this
race causing bugs due to removal of in-use modules.  However there are
other real bugs in module.c code and driver code (module_refcount is
exported) where the callers do not run under stop_machine.

Fix this by maintaining running per-cpu counters for the number of
module refcount increments and the number of refcount decrements.  The
increments are tallied after the decrements, so any decrement seen will
always have its corresponding increment counted.  The final refcount is
the difference of the total increments and decrements, preventing a
low-refcount from being returned.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>