msm: kgsl: Check for integer overflow before allocating memory

In _kgsl_sharedmem_page_alloc(), if the requested size is very
large then there could be a possibility of integer overflow
during memory allocation for scatterlist.

To fix this, check for integer overflow condition before
allocating the memory.

CRs-Fixed: 1005344
Change-Id: I6e0015ab67d6b13fc94d94dafec41788f53820d3
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>