Description: fork: fix copy_process(CLONE_PARENT) race with the exiting ->real_parent
References:
 https://bugzilla.redhat.com/show_bug.cgi?id=1902724
Notes:
Bugs:
upstream: released (5.10-rc3) [b4e00444cab4c3f3fec876dc0cccc8cbb0d1a948]
5.10-upstream-stable: N/A "Fixed before branching point"
4.19-upstream-stable: released (4.19.156) [b177d2d915cea2d0a590f0034a20299dd1ee3ef2]
4.9-upstream-stable: released (4.9.242) [66be43d81870c55637c2f32d8088d7184e93262a]
sid: released (5.9.9-1)
4.19-buster-security: released (4.19.160-1)
4.9-stretch-security: released (4.9.246-1)