notifier: Return an error when a callback has already been registered

Return -EEXIST when a notifier callback has already been registered on a
notifier chain.

This should avoid any homegrown registration tracking at the callsite
like

  https://lore.kernel.org/amd-gfx/[email protected]

for example.

This version is an alternative of

  https://lore.kernel.org/r/[email protected]

which needed to touch every caller not checking the registration
routine's return value.

Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Sebastian Andrzej Siewior <[email protected]>
Acked-by: Alan Stern <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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