arm64/mte: Remove asymmetric mode from the prctl() interface

As pointed out by Evgenii Stepanov one potential issue with the new ABI for
enabling asymmetric is that if there are multiple places where MTE is
configured in a process, some of which were compiled with the old prctl.h
and some of which were compiled with the new prctl.h, there may be problems
keeping track of which MTE modes are requested. For example some code may
disable only sync and async modes leaving asymmetric mode enabled when it
intended to fully disable MTE.

In order to avoid such mishaps remove asymmetric mode from the prctl(),
instead implicitly allowing it if both sync and async modes are requested.
This should not disrupt userspace since a process requesting both may
already see a mix of sync and async modes due to differing defaults between
CPUs or changes in default while the process is running but it does mean
that userspace is unable to explicitly request asymmetric mode without
changing the system default for CPUs.

Reported-by: Evgenii Stepanov <[email protected]>
Signed-off-by: Mark Brown <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Reviewed-by: Evgenii Stepanov <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Cc: Joey Gouly <[email protected]>
Cc: Branislav Rankov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

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