selinux: fix potential counting error in avc_add_xperms_decision()

The count increases only when a node is successfully added to
the linked list.

Cc: stable@vger.kernel.org
Fixes: fa1aa143 ("selinux: extended permissions for ioctls")
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>