mds: touch missed dentry when fetching dir on path traverse

mds: touch missed dentry when fetching dir on path traverse

We can get into a loop when doing a path traverse if we miss on a large
directory and then end up trimming the result we need before handling the
original request.  To avoid this, we simply put the wanted dentry at the
top of the LRU (instead of midpoint).

Signed-off-by: Sage Weil <sage@newdream.net>