parasite: Always restore thread context in parasite_ctl_fini

The sigreturn should not be treated as doing all work for us,
we can fail at eny moment, so just call ptrace_set_thread_ctx
at the exit -- this restores registers back and unmask blocked
signals (just like CRIU does).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>