diag: Allocate DCI memory using vzalloc instead of kzalloc

Currently there is a possibility of kmalloc failing
when system is running low on memory condition.
The patch changes the dci memory allocation from
kzalloc to vzalloc.

CRs-Fixed: 2195818
Change-Id: I92b20d8e77ce5b2a96212f9d0757fbbff2703891
Signed-off-by: Hardik Arya <harya@codeaurora.org>