restore: define root_as_sibling before using it

root_as_sibling is used in criu_signals_setup(), but was only defined later
(when forking the root task for the first time). This meant that the
SA_NOCLDSTOP was never masked off, which meant SIGCHLD was never delivered
after ptracing the root task. Thus, when the a child of the root task died
(e.g. from cr_system), the root task sat in PTRACE_STOP, and the restore task
never PTRACE_CONT'd, resulting in a deadlock.

v2: re-work the condition for CLONE_PARENT
v3: move unmasking of SA_NOCLDSTOP to restore_root_task

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>