videobuf2-core: Use current->mm_active when locking USERPTR

The vb2 framework currently assumes current has a real address space.
However, it is possible for anonymous processes with no real address
space to use videobuf2 framework.  For anonymous processes
current->active_mm should be used rather than current->mm, which is not
valid.  current->active_mm can also be used for non-anonymous processes
as it would be equivalent to current->mm.

See Documentation/vm/active_mm.txt for further info.

Change-Id: I1b6283a8368af41b47086ef8b40c8a00a6859911
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>