seq_file: return a negative error code when seq_path_root() fails.

seq_path_root() is returning a return value of successful __d_path()
instead of returning a negative value when mangle_path() failed.

This is not a bug so far because nobody is using return value of
seq_path_root().

Signed-off-by: Tetsuo Handa <[e-mail]>
Cc: Al Viro <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/seq_file.c

···
 				m->count = s - m->buf;
 				return 0;
 			}
+			err = -ENAMETOOLONG;
 		}
 	}
 	m->count = m->size;