[media] videobuf2-core: Avoid locking userpages if no current->mm

If vb2_qbuf is called within the context of a kthread with no "user
memory map", current->mm will be NULL causing a fault when attempt to
lock the non-existent pages.

As such, perform a cursory check for a non-NULL current->mm before
locking the userspace pages.

CRs-Fixed: 623719
Change-Id: I138ba4d1611fae821e723a11b01046e5fc4f9092
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>