Don't allow the stack to grow into hugetlb reserved regions (CVE-2007-3739)

When expanding the stack, we don't currently check if the VMA will cross
into an area of the address space that is reserved for hugetlb pages.
Subsequent faults on the expanded portion of such a VMA will confuse the
low-level MMU code, resulting in an OOPS.  Check for this.

Signed-off-by: Adam Litke <agl@us.ibm.com>
Signed-off-by: Adrian Bunk <bunk@kernel.org>