coresight: etb10: Fix possible NULL ptr dereference in etb_enable_perf()

There was a report of NULL pointer dereference in ETF enable
path for perf CS mode with PID monitoring. It is almost 100%
reproducible when the process to monitor is something very
active such as chrome and with ETF as the sink, not ETR.

But code path shows that ETB has a similar path as ETF, so
there could be possible NULL pointer dereference crash in
ETB as well. Currently in a bid to find the pid, the owner
is dereferenced via task_pid_nr() call in etb_enable_perf()
and with owner being NULL, we can get a NULL pointer
dereference, so have a similar fix as ETF where we cache PID
in alloc_buffer() callback which is called as the part of
etm_setup_aux().

Fixes: 75d7dbd38824 ("coresight: etb10: Add support for CPU-wide trace scenarios")
Cc: stable@vger.kernel.org
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20201127175256.1092685-11-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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