Description: gup: document and work around "COW can break either way" issue
References:
 https://bugs.chromium.org/p/project-zero/issues/detail?id=2045
 https://lore.kernel.org/stable/20210401182125.171484-1-surenb@google.com/
 https://lore.kernel.org/stable/20211012015244.693594-1-surenb@google.com/
Notes:
 bwh> The issue is said to go back to "2.x kernels"
 carnil> The backport for 4.9.y got reverted in 4.9.298, cf.
 carnil> 6fbb8383884f2c89f4c7e2c8603b5ed1b90b815f, and then followed by
 carnil> 0c29640bdecad332b9e2b884217c159f4aeb2556.
Bugs:
upstream: released (5.8-rc1) [17839856fd588f4ab6b789f482ed3ffd7c403e1f]
5.10-upstream-stable: N/A "Fixed before branch point"
4.19-upstream-stable: released (4.19.189) [5e24029791e809d641e9ea46a1f99806484e53fc], released (4.19.226) [294c7a9fb608c29a9e49010b515228e20ccbec8f]
4.9-upstream-stable: released (4.9.298) [0c29640bdecad332b9e2b884217c159f4aeb2556]
sid: released (5.7.6-1)
5.10-bullseye-security: N/A "Fixed before branching point"
4.19-buster-security: released (4.19.194-1), released (4.19.232-1)
4.9-stretch-security: released (4.9.272-1) [bugfix/all/gup-document-and-work-around-cow-can-break-either-wa.patch]