signal/ptrace: fix cgroup2/freezer long runtimes

As reported in the thread: https://lkml.org/lkml/2019/10/1/789

, in
kernels with commit 76f969e8 [cgroup: cgroup v2 freezer], we
were seeing much longer runtime in strace/ptrace tests (4 minutes
versus 4 seconds).

The issue only manifests if CONFIG_PREEMPT is enabled, which is in
all of the default configurations.

As sugggested in the thread, the movement of preempt_enable_no_resched()
until after the cgroup is frozen returns the behaviour to pre-5.2
runtimes.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>