mds: check dn->last when finding existing dentries during replay

mds: check dn->last when finding existing dentries during replay

We can't simply search for an existing dentry based on the name and end
snap, as that may turn up the wrong item.  For example, if we have
[2,head] and the replaying operations cowed that to [2,2] and [3,head], then
if we replay the [2,2] item first we'll find [2,head] (the _wrong_ dentry)
and throw an assertion.

So just check for dn->last != p->dnlast.