diag: Allocate mask update buffer using vzalloc

With possibility of unavailable pages of higher order, memory
allocation using kmalloc can fail. Use vzalloc to prevent
memory allocation failure.

Change-Id: I94de8ada3ccadd7868d2e2688fd0fa25a5f42c8a
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>