mds: don't overwrite reply's snapbl

set_trace_dist() updates reply's snapbl, don't overwrite it. For MKSNAP
request, just need to set mdr->tracei, set_trace_dist() will set reply's
snapbl.

Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>