mm: usercopy: skip stack page span check

Kernel stack pages can span multiple pages and GFP_COMP is not
set for stack pages. Fix the false "spans multiple pages" error.

Change-Id: I2ba414be466b7c90fb4a3542237da809175e0590
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
[swatsrid@codeaurora.org: Fix trivial merge conflicts]
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>