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: I8e18aad2118f58ce677050ff4c4a4b0823c4b4b3