ion: validate ion client in ion_debug_client_show() and extract data

ion client can get created and destroyed dynamically.
Client data struct may not be in proper state if one core is reading
client information and other core is destroying the same client.
There is no synchronization between ion_debug_client_show() and
ion_client_destroy(), that leaves client data structures in
uncertain state and which may lead to system crash.

So add a check in ion_debug_client_show(), to verify whether the client
is part of ion device rb tree or not before retrieving client data.

CRs-Fixed: 743714
Change-Id: Ic4c2f9dff905d9bb8fbf2e56b7c6396dc01bdf8f
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>