staging: android: ion: check for kref overflow

This patch is against 4.4. It does not apply to master due to a large
rework of ion in 4.12 which removed the affected functions altogther.
4c23cbff ("staging: android: ion: Remove import interface")

Userspace can cause the kref to handles to increment
arbitrarily high. Ensure it does not overflow.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>