memshare: Correct the usage of index and client_id

Array index of memblock and the client_id of memshare clients
confused usage can cause issue. Correct the usage by properly
defining variable names.

Change-Id: I29a0c6a01d63e92774e11fd258d018c03769ca50
Signed-off-by: Manoj Prabhu B <bmanoj@codeaurora.org>