lib: genalloc: Change chunk allocation to vmalloc

Change the chunk allocation from kmalloc to vmalloc for
allocations greater than a page. This allows large
chunks to be allocated from physically non-contiguous
memory and increases the chance of the allocation
succeeding.

CRs-fixed: 387655
Change-Id: I75cd0d1634c6f8ff4d91e615122fdcfada00ec69
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>