Blackfin: don't walk VMAs when oopsing

If we're double faulting, then we have to assume the VMAs are not safe as
broken pointers here will prevent full trace output for the double fault.
Shouldn't be a big problem though as rarely is a double fault caused by
code in userspace.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>