audit: add lock for audit_sock

Netlink is an asyncchronous communication, and audit_sock
is a global variable, so it is necessary to add a lock when
using audit_sock.

Change-Id: I4f8e8ecde7dae3b12a6e73e9c1f1b358812d050a
Signed-off-by: Teng Fei Fan <tengfei@codeaurora.org>
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>