nfs: we don't support removing system.nfs4_acl

The NFSv4 protocol doesn't have any notion of reomoving an attribute, so
removexattr(path,"system.nfs4_acl") doesn't make sense.

There's no documented return value.  Arguably it could be EOPNOTSUPP but
I'm a little worried an application might take that to mean that we
don't support ACLs or xattrs.  How about EINVAL?
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs4proc.c

@@ -5889,6 +5889,9 @@ static int __nfs4_proc_set_acl(struct inode *inode, const void *buf, size_t bufl
 	unsigned int npages = DIV_ROUND_UP(buflen, PAGE_SIZE);
 	int ret, i;
 
+	/* You can't remove system.nfs4_acl: */
+	if (buflen == 0)
+		return -EINVAL;
 	if (!nfs4_server_supports_acls(server))
 		return -EOPNOTSUPP;
 	if (npages > ARRAY_SIZE(pages))