mds/Server: return after responding on error paths

handle_client_symlink (-ENAMETOOLONG), reclaim_session (-EPERM) and
handle_client_readdir_snapdiff (-EINVAL) each sent an error reply on a
failure path but fell through, replying to the same MDRequest a second
time. reclaim_session additionally went on to bind reclaiming_from,
bypassing the auth_name check. Return right after sending the reply.

Fixes: https://tracker.ceph.com/issues/77862
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>