ARM: 6052/1: kdump: make kexec work in interrupt context

When crash happens in interrupt context there is no userspace context.
We always use current->active_mm in those cases.

Signed-off-by: Mika Westerberg <ext-mika.1.westerberg@nokia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>