nfs_common: need lock during iterate through the list

If the elem is deleted during be iterated on it, the iteration
process will fall into an endless loop.

kernel: NMI watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [nfsd:17137]

PID: 17137  TASK: ffff8818d93c0000  CPU: 4   COMMAND: "nfsd"
    [exception RIP: __state_in_grace+76]
    RIP: ffffffffc00e817c  RSP: ffff8818d3aefc98  RFLAGS: 00000246
    RAX: ffff881dc0c38298  RBX: ffffffff81b03580  RCX: ffff881dc02c9f50
    RDX: ffff881e3fce8500  RSI: 0000000000000001  RDI: ffffffff81b03580
    RBP: ffff8818d3aefca0   R8: 0000000000000020   R9: ffff8818d3aefd40
    R10: ffff88017fc03800  R11: ffff8818e83933c0  R12: ffff8818d3aefd40
    R13: 0000000000000000  R14: ffff8818e8391068  R15: ffff8818fa6e4000
    CS: 0010  SS: 0018
 #0 [ffff8818d3aefc98] opens_in_grace at ffffffffc00e81e3 [grace]
 Kwiboo#1 [ffff8818d3aefca8] nfs4_preprocess_stateid_op at ffffffffc02a3e6c [nfsd]
 Kwiboo#2 [ffff8818d3aefd18] nfsd4_write at ffffffffc028ed5b [nfsd]
 Kwiboo#3 [ffff8818d3aefd80] nfsd4_proc_compound at ffffffffc0290a0d [nfsd]
 Kwiboo#4 [ffff8818d3aefdd0] nfsd_dispatch at ffffffffc027b800 [nfsd]
 Kwiboo#5 [ffff8818d3aefe08] svc_process_common at ffffffffc02017f3 [sunrpc]
 Kwiboo#6 [ffff8818d3aefe70] svc_process at ffffffffc0201ce3 [sunrpc]
 Kwiboo#7 [ffff8818d3aefe98] nfsd at ffffffffc027b117 [nfsd]
 Kwiboo#8 [ffff8818d3aefec8] kthread at ffffffff810b88c1
 Kwiboo#9 [ffff8818d3aeff50] ret_from_fork at ffffffff816d1607

The troublemake elem:
crash> lock_manager ffff881dc0c38298
struct lock_manager {
  list = {
    next = 0xffff881dc0c38298,
    prev = 0xffff881dc0c38298
  },
  block_opens = false
}

Fixes: c87fb4a ("lockd: NLM grace period shouldn't block NFSv4 opens")
Signed-off-by: Cheng Lin <cheng.lin130@zte.com.cn>
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfs_common/grace.c

@@ -69,10 +69,14 @@ __state_in_grace(struct net *net, bool open)
 	if (!open)
 		return !list_empty(grace_list);
 
+	spin_lock(&grace_lock);
 	list_for_each_entry(lm, grace_list, list) {
-		if (lm->block_opens)
+		if (lm->block_opens) {
+			spin_unlock(&grace_lock);
 			return true;
+		}
 	}
+	spin_unlock(&grace_lock);
 	return false;
 }