fortify: Provide a memcpy trap door for sharp corners

As we continue to narrow the scope of what the FORTIFY memcpy() will
accept and build alternative APIs that give the compiler appropriate
visibility into more complex memcpy scenarios, there is a need for
"unfortified" memcpy use in rare cases where combinations of compiler
behaviors, source code layout, etc, result in cases where the stricter
memcpy checks need to be bypassed until appropriate solutions can be
developed (i.e. fix compiler bugs, code refactoring, new API, etc). The
intention is for this to be used only if there's no other reasonable
solution, for its use to include a justification that can be used
to assess future solutions, and for it to be temporary.

Example usage included, based on analysis and discussion from:
https://lore.kernel.org/netdev/CANn89iLS_2cshtuXPyNUGDPaic=sJiYfvTb_wNLgWrZRyBxZ_g@mail.gmail.com

Cc: Jakub Kicinski <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Coco Li <[email protected]>
Cc: Tariq Toukan <[email protected]>
Cc: Saeed Mahameed <[email protected]>
Cc: Leon Romanovsky <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

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