snapshot: replace global sync with a namespace sync

snapshot: replace global sync with a namespace sync

snapshot_create() called did a global 'sync' after freeze()
which syncs everything including all other containers and
the host. So if you want to snapshot container A while
container B tries to write to a broken NFS mount the
snapshot will hang in that sync call.
Instead we now enter the container's mount namespace and do
a syncfs() on all of its mountpoints.