Various archs (i386, m68k, s390, s390x, m68k, parisc, um, x86_64)
implement kernel_text_address.  Put this in kernel/extable.c, and the
module iteration inside module.c.

Other than cleanliness, this finally allows the module list and lock
to be static to kernel/module.c (you didn't think I actually cared
about those archs did you?)

It also drops the module->init_size to zero when it's discarded, so
bounds checking is simplified (and the /proc/modules size statistic
will be more accurate, too).