msm: kgsl: Add missing check for alloc size(CVE-2016-2468)

In _kgsl_sharedmem_page_alloc(), check for boundary limits
of requested alloc size before honoring.

Change-Id: Ib76926a6c5994065d5c8f4a9e36b34dff5d4596b
Reviewed-on: http://mcrd1-22-pc.corpnet.asus/code-review/master/238963


Reviewed-by: shunmin_wang <shunmin_wang@asus.com>
Tested-by: shunmin_wang <shunmin_wang@asus.com>
Reviewed-by: Carol_Jiang <carol_jiang@asus.com>