SELinux: Fix RCU deref check warning in sel_netport_insert()

Fix the following bug in sel_netport_insert() where rcu_dereference() should
be rcu_dereference_protected() as sel_netport_lock is held.

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
security/selinux/netport.c:127 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 0
1 lock held by ossec-rootcheck/3323:
 #0:  (sel_netport_lock){+.....}, at: [<ffffffff8117d775>] sel_netport_sid+0xbb/0x226

stack backtrace:
Pid: 3323, comm: ossec-rootcheck Not tainted 3.1.0-rc8-fsdevel+ #1095
Call Trace:
 [<ffffffff8105cfb7>] lockdep_rcu_dereference+0xa7/0xb0
 [<ffffffff8117d871>] sel_netport_sid+0x1b7/0x226
 [<ffffffff8117d6ba>] ? sel_netport_avc_callback+0xbc/0xbc
 [<ffffffff8117556c>] selinux_socket_bind+0x115/0x230
 [<ffffffff810a5388>] ? might_fault+0x4e/0x9e
 [<ffffffff810a53d1>] ? might_fault+0x97/0x9e
 [<ffffffff81171cf4>] security_socket_bind+0x11/0x13
 [<ffffffff812ba967>] sys_bind+0x56/0x95
 [<ffffffff81380dac>] ? sysret_check+0x27/0x62
 [<ffffffff8105b767>] ? trace_hardirqs_on_caller+0x11e/0x155
 [<ffffffff81076fcd>] ? audit_syscall_entry+0x17b/0x1ae
 [<ffffffff811b5eae>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff81380d7b>] system_call_fastpath+0x16/0x1b

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: stable@kernel.org
Signed-off-by: James Morris <jmorris@namei.org>