arm64/mte: Clarify mode reported by PR_GET_TAGGED_ADDR_CTRL

arm64/mte: Clarify mode reported by PR_GET_TAGGED_ADDR_CTRL

With the current wording readers might infer that PR_GET_TAGGED_ADDR_CTRL
will report the mode currently active in the thread however this is not the
actual behaviour, instead all modes currently selected by the process will
be reported with the mode used depending on the combination of the
requested modes and the default set for the current CPU. This has been the
case since be4b925d276f55 ("arm64: mte: change ASYNC and SYNC TCF settings
into bitfields"), before that we did not allow more than one mode to be
requested simultaneously.

Update the documentation to more clearly reflect current behaviour.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20220127190324.660405-1-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>