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"
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]
4.9-upstream-stable: released (4.9.287) 9bbd42e79720122334226afad9ddcac1c3e6d373]
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)
4.9-stretch-security: released (4.9.272-1) [bugfix/all/gup-document-and-work-around-cow-can-break-either-wa.patch]