ion:synchronize debugfs callback and ion_client_destroy

There are race condition B/T ion_client_destroy and debugfs callbacks.
Let's use a mutex to synchronize them.

Change-Id: I3373dc1dbb551b615105a485cc2d3c4bcc0e5e99
Signed-off-by: Neil Zhang <neilzhang1123@hotmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 948c4db4ee10d85fe78ed3755dcaeb85cd37a148
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[guptap@codeaurora.org: resolve trivial merge conflicts and
 change usage of %p with %pK]
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>