NFS: Guard against READDIR loop when entry names exceed MAXNAMELEN

Commit 64cfca85 asserts the only valid return values for
nfs2/3_decode_dirent should not include -ENAMETOOLONG, but for a server
that sends a filename3 which exceeds MAXNAMELEN in a READDIR response the
client's behavior will be to endlessly retry the operation.

We could map -ENAMETOOLONG into -EBADCOOKIE, but that would produce
truncated listings without any error.  The client should return an error
for this case to clearly assert that the server implementation must be
corrected.

Fixes: 64cfca85 ("NFS: Return valid errors from nfs2/3_decode_dirent()")
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/nfs2xdr.c

@@ -949,7 +949,7 @@ int nfs2_decode_dirent(struct xdr_stream *xdr, struct nfs_entry *entry,
 
 	error = decode_filename_inline(xdr, &entry->name, &entry->len);
 	if (unlikely(error))
-		return -EAGAIN;
+		return error == -ENAMETOOLONG ? -ENAMETOOLONG : -EAGAIN;
 
 	/*
 	 * The type (size and byte order) of nfscookie isn't defined in

fs/nfs/nfs3xdr.c

@@ -1991,7 +1991,7 @@ int nfs3_decode_dirent(struct xdr_stream *xdr, struct nfs_entry *entry,
 
 	error = decode_inline_filename3(xdr, &entry->name, &entry->len);
 	if (unlikely(error))
-		return -EAGAIN;
+		return error == -ENAMETOOLONG ? -ENAMETOOLONG : -EAGAIN;
 
 	error = decode_cookie3(xdr, &new_cookie);
 	if (unlikely(error))