diag: Add kmemleak_not_leak checks to avoid memleaks

Add kmemleak_not_leak call in diagmem_alloc to avoid
memleaks since these are not leak cases.

Change-Id: I003d7b4f5bd1492f649031056c550b3633ff3036
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>