msm: kgsl: Fix a sign mismatch in memdesc_sg_virt

get_user_pages returns a signed long but we do all of our math
with unsigned longs so cast the return value to the appropriate
type for a comparison.

Change-Id: Ic0dedbad0c931c2440929c615879a288d2f13828
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>