sched: walt: Acquire rq-locks in update_min_max_capacity

Previously, the function named in the subject line was relying on
stubbed out functions to ensure rq locks were taken during cluster
capacity updates. Fix this.

Change-Id: I272d8edb0c22046088cd2fdbaefade0174801c1c
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>