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 <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>