sched: walt: fix out-of-bounds access

A computation in update_top_tasks() is indexing
off the end of a top_tasks array. There's code
to limit the index in the computation, but it's
insufficient.

Bug: 110529282
Change-Id: Idb5ff5e5800c014394bcb04638844bf1e057a40c
Signed-off-by: John Dias <joaodias@google.com>
[pkondeti@codeaurora.org: Backported to 4.4 for HMP scheduler]
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>