ceph: fix dn offset during readdir_prepopulate

When adding the readdir results to the cache, ceph_set_dentry_offset was
clobbered our just-set offset.  This can cause the readdir result offsets
to get out of sync with the server.  Add an argument to the helper so
that it does not.

This bug was introduced by 1cd3935b.

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