Blackfin: fix up mm locking in address dumping

The locking code in the address dumper needs to grab the mm's mmap_sem
so that other CPUs do not get an inconsistent view.  On UP systems this
really wasn't a problem, but it is easy to trigger a race on SMP systems
when another CPU removes a mapping.

Signed-off-by: Graf Yang <graf.yang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>