NFSv4: Fix an Oops during delegation callbacks

If the server sends a CB_GETATTR or a CB_RECALL while the filesystem is
being unmounted, then we can Oops when releasing the inode in
nfs4_callback_getattr() and nfs4_callback_recall().
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/callback_proc.c

@@ -66,7 +66,7 @@ __be32 nfs4_callback_getattr(void *argp, void *resp,
 out_iput:
 	rcu_read_unlock();
 	trace_nfs4_cb_getattr(cps->clp, &args->fh, inode, -ntohl(res->status));
-	iput(inode);
+	nfs_iput_and_deactive(inode);
 out:
 	dprintk("%s: exit with status = %d\n", __func__, ntohl(res->status));
 	return res->status;
@@ -108,7 +108,7 @@ __be32 nfs4_callback_recall(void *argp, void *resp,
 	}
 	trace_nfs4_cb_recall(cps->clp, &args->fh, inode,
 			&args->stateid, -ntohl(res));
-	iput(inode);
+	nfs_iput_and_deactive(inode);
 out:
 	dprintk("%s: exit with status = %d\n", __func__, ntohl(res));
 	return res;

fs/nfs/delegation.c

@@ -850,16 +850,23 @@ nfs_delegation_find_inode_server(struct nfs_server *server,
 				 const struct nfs_fh *fhandle)
 {
 	struct nfs_delegation *delegation;
-	struct inode *res = NULL;
+	struct inode *freeme, *res = NULL;
 
 	list_for_each_entry_rcu(delegation, &server->delegations, super_list) {
 		spin_lock(&delegation->lock);
 		if (delegation->inode != NULL &&
 		    nfs_compare_fh(fhandle, &NFS_I(delegation->inode)->fh) == 0) {
-			res = igrab(delegation->inode);
+			freeme = igrab(delegation->inode);
+			if (freeme && nfs_sb_active(freeme->i_sb))
+				res = freeme;
 			spin_unlock(&delegation->lock);
 			if (res != NULL)
 				return res;
+			if (freeme) {
+				rcu_read_unlock();
+				iput(freeme);
+				rcu_read_lock();
+			}
 			return ERR_PTR(-EAGAIN);
 		}
 		spin_unlock(&delegation->lock);