rbd-mirror: allow removal of user group snapshot if id do not match

rbd-mirror: allow removal of user group snapshot if id do not match

Currently in the unlink_group_snapshots() we check for regular group snapshot
names on remote group and local group. If the same name user snapshot exists on
remote group we do not remove it locally. There might be a case, user deleted
the user group snapshot on the remote group and recreated it with the same name
again (before the delete update via a mirror group snapshot is conveyed locally),
even in such case unfortunately currently we still don't delete the group
snapshot locally as the names still matching with the locally existing snapshot.
Hence we need to preserve the user snapshot locally only if their id's match and
not the group snapshot names.

Without this fix the mirroring daemon stays stuck at the same snapshot.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>