remap: Keep nsid on remap rollback, not the ns' pid

This is more logical -- we should remap file in namespace,
not in pid. Also this would help to clean the code further.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mnt: Turn mntns_get_root_fd into accepting mnt ns_id

The only exception (for now) is the irmap -- it should
operate on ns as well.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>