gpu: ion: Map only the vma size given

When mapping carveout buffers into userspace, only map
the size of the vma given, not the full size of the buffer
since clients may map less than the buffer size.

Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>