PG:: filter_snapc: Return immediately if no snapc need to trim
we can return immediately if no snapc need to trim. Do not iterater snapc vector and do extra judgement and ops.

Signed-off-by: Ning Yao <zay11022@gmail.com>
Merge pull request #3257 from mslovy/wip-y02

PG:: filter_snapc: Return immediately if no snapc need to trim

Reviewed-by: Sage Weil <sage@redhat.com>