i915/selftest/igt_mmap: let mmap tests run in kthread

When the driver is loaded on the system with numa nodes it might be run in
a kthread, which makes it impossible to use current->mm in the selftest.
This patch allows the selftest to use current->mm by using active_mm.

Signed-off-by: Mikolaj Wasiak <mikolaj.wasiak@intel.com>
Reviewed-by: Eugene Kobyak <eugene.kobyak@intel.com>
Reviewed-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2w6pt2hnemndwmanwhyn3keexa6vtha7rmo6rqoerkmyxhbrh2@ls7lndjpia6z