ReplicatedPG: make_snap_collection when moving snap link in snap_trimmer

Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 88956e3186798058a1170803f8abfc0f3cf77a07)