sched: walt: fix use after free in walt_task_dump()

When SCHED_BUG_ON() is hit, we are printing the current tasks of
all online CPUs. Accessing the task on the remote CPU is racy. It
can be made safe by incrementing the task struct reference and also
adding a check against the exiting flag. The WALT CPU load pointers
are freed only after setting the exiting flag. This exiting flag
check helps is not accessing the freed memory.

Change-Id: I2fcbd37e9e4f096da5372be041aec2b60da26341
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>