msm: kgsl: Add missing checks for alloc size and sglen

In _kgsl_sharedmem_page_alloc(), check for boundary limits
of requested alloc size before honoring and make sure sglen
is greater than zero before marking it as end of sg list.

Change-Id: I8b9e225e515a0f31593df6f4cad253236475d0ae
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>