unshare: Fix PDEATHSIG race for --kill-child

unshare: Fix PDEATHSIG race for --kill-child

Kill the child explicitly should the parent terminate
just before the child invokes prctl(PR_SET_PDEATHSIG).

The underlying issue can be reproduced as follows:

  #!/bin/bash

  rm -f /tmp/unshare.log.*
  strace -ff -tt -o /tmp/unshare.log /usr/bin/unshare --kill-child --fork true
  head -1 /tmp/unshare.log.*

  while : ; do
    PARENT=$(
      /usr/bin/unshare --kill-child --fork bash -c 'echo $PPID' &
      sleep 0 # sleep 0.00$RANDOM
      kill -9 $! 2>/dev/null
    )
    [ "$PARENT" != 1 ] || { echo "$PARENT" ; break ; }
  done

Signed-off-by: Earl Chew <earl_chew@yahoo.com>