perf: Cancel the mux hrtimer during CPU hotplug to avoid migration

The current design of hrtimers migrates the pinned timers to a
different CPU upon its hotplug. However, perf-core needs to
maintain the mux-hrtimers on a per CPU basis. That is, each
hrtimer carries the context for that particular CPU and would
lose this context if it gets migrated to a different CPU. As a
result, cancel the hrtimer for the CPU that's about to go down
and restart it (if required) when the perf-events are being created.

Change-Id: I7a1d0456208855e3a99a7d49e59c6dae811d146e
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
[mojha@codeaurora.org: Resolved merge conflict and added missing
 `cpuctx` variable to avoid build failure]
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>