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

In _kgsl_sharedmem_page_alloc():

- Make len of type size_t to be in line with size.
- Check for boundary limits of requested alloc size before honoring.
- Make sure sglen is greater than zero before marking it as end
of sg list.

Bug: 27475454
Change-Id: I5b2e6f657f532fc256627cb6b2ab3ca01938a11b
Signed-off-by: Yuan Lin <yualin@google.com>