arm64: mte: Fix PTRACE_{PEEK,POKE}MTETAGS error documentation

PTRACE_{PEEK,POKE}MTETAGS return -EIO rather than -EOPNOTSUPP (as
documented) when no tags are copied from/to a mapping without PROT_MTE.
This has been the behaviour since the interface was introduced, though
the original intent was to distinguish between address not being
accessible and mapped as untagged.

Update the documentation to match the implementation (de-facto ABI).
Since -EOPNOTSUPP was never returned, change the error assignment to
-EIO as well to avoid confusion.

Fixes: df9d7a22dd21 ("arm64: mte: Add Memory Tagging Extension documentation")
Fixes: 18ddbaa02b7a ("arm64: mte: ptrace: Add PTRACE_{PEEK,POKE}MTETAGS support")
Reported-by: Yury Khrustalev <yury.khrustalev@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>