mds/server: skip unwanted dn in handle_client_readdir

mds/server: skip unwanted dn in handle_client_readdir

We can skip unwanted dn which  < (offset_key, snap) via map.lower_bound, rather than
iterate across them.

Previously we iterate and skip dn which < (offset_key, dn->last), as dn->last >= snap
 means (offset_key, dn->last) >= (offset_key, snap), and such iterate_and_skip logic
still keep, so this commit doesnt change code logic but an optimization.

Signed-off-by: Xiaoxi Chen <xiaoxchen@ebay.com>
(cherry picked from commit 52fe52baf920c672ac7f63a3087dcd31137891b6)