Revert "[media] videobuf2-core: Use current->mm_active when locking USERPTR"

Reverting commit 705d2156b85febeb47509e9d955052790b9025e0 because using
mm_active seems too unreliable. When current is a kthread, there's no
guarantee that there will not be a context switch between down_read and
up_read, which causes a panic due to mismatched up/down_reads.

Change-Id: Ibfb1ad81fbe30be88ccc5f38482578fee09dd83a
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>