NFS: Return -ETXTBSY when attempting to write to a swapfile

My understanding is that -EBUSY refers to the underlying device, and
that -ETXTBSY is used when attempting to access a file in use by the
kernel (like a swapfile). Changing this return code helps us pass
xfstests generic/569

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/file.c

@@ -649,7 +649,7 @@ out:
 
 out_swapfile:
 	printk(KERN_INFO "NFS: attempt to write to active swap file!\n");
-	return -EBUSY;
+	return -ETXTBSY;
 }
 EXPORT_SYMBOL_GPL(nfs_file_write);