NFS: Add an asynchronous delegreturn operation for use in nfs_clear_inode

Otherwise, there is a potential deadlock if the last dput() from an NFSv4
close() or other asynchronous operation leads to nfs_clear_inode calling
the synchronous delegreturn.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/inode.c

@@ -1145,7 +1145,7 @@ static int nfs_update_inode(struct inode *inode, struct nfs_fattr *fattr)
 void nfs4_clear_inode(struct inode *inode)
 {
 	/* If we are holding a delegation, return it! */
-	nfs_inode_return_delegation(inode);
+	nfs_inode_return_delegation_noreclaim(inode);
 	/* First call standard NFS clear_inode() code */
 	nfs_clear_inode(inode);
 }