If processes share a file descriptor table, they all have their own set
of service descriptors. close_old_fds() closes all file descriptors
execpt service descriptors

Fixes: 9d60724e ("restore: restore mntns before creating private vma-s (v3)")
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>