Description: relay: handle alloc_percpu returning NULL in relay_open
References:
 https://lore.kernel.org/lkml/20191129013745.7168-1-dja@axtens.net/
 https://syzkaller-ppc64.appspot.com/bug?id=1c09906c83a8ea811a9e318c2a4f8e243becc6f8
 https://syzkaller-ppc64.appspot.com/bug?id=b05b4d005191cc375cdf848c3d4d980308d50531
 https://syzkaller.appspot.com/bug?id=e4265490d26d6c01cd9bc79dc915ef0a1bf15046
 https://syzkaller.appspot.com/bug?id=f4d1cb4330bd3ddf4a628332b4285407b2eedd7b 
Notes:
 bwh> Introduced in 4.9 (not 4.10) by commit 017c59c042d0 "relay: Use per
 bwh> CPU constructs for the relay channel buffer pointers".
Bugs:
upstream: needed
4.19-upstream-stable: needed
4.9-upstream-stable: needed
3.16-upstream-stable: N/A "Vulnerability introduced later"
sid: needed
4.19-buster-security: needed
4.9-stretch-security: needed
3.16-jessie-security: N/A "Vulnerability introduced later"