kclient: fix di->off calculation

kclient: fix di->off calculation

The dentry dir offset calculation wasn't taking into account the
possibility of multiple readdi requests, which in turn meant bad results
for readdir-from-dcache.

Since doing this on the client side was a mess, the MDS includes a dentry
offset for each readdir dentry within the dirfrag.  This value is stored
in di->offset (with adjustment in leftmost frag for . and ..), and that's
the value that's passed back via filldir.