ANDROID: netfilter: xt_qtaguid: fix a race condition in if_tag_stat_update

Miss a lock protection in if_tag_stat_update while doing get_iface_entry. So if
one CPU is doing iface_stat_create while another CPU is doing if_tag_stat_update,
race will happened.

Change-Id: Ib8d98e542f4e385685499f5b7bb7354f08654a75
Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>