mds: make new inodes follow dn->first

mds: make new inodes follow dn->first

We can get a dn->first that is greater than the parent dir's seq.  Notably,
when we do something like

 mkdir foo
 mkdir foo/.snap/a
 rmdir foo/.snap/a
 rmdir foo

Here, the foo dentry has a high seq, and subsequent mkdir foo should make
sure we give the new foo dir inode the same seq (and not a lower one from
the parent).  Otherwise, things get confused later on.