BACKPORT: staging: ion: Prevent incorrect reference counting behavour

Supply additional check in order to prevent unexpected results.

Bug: 205573273
Fixes: b892bf75 ("ion: Switch ion to use dma-buf")
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[Lee: Patch now applies to ion_buffer.c instead of ion.c]
Change-Id: Ia6afdd9ca502caa9cad6619d438fc6c8e8457679
(cherry picked from commit 27da8d16)