arm64/sme: Optimise SME exit on syscall entry

Our ABI says that we exit streaming mode on syscall entry. Currently we
check if we are in streaming mode before doing this but since we have a
SMSTOP SM instruction which will clear SVCR.SM in a single atomic operation
we can save ourselves the read of the system register and check of the flag
and just unconditionally do the SMSTOP SM. If we are not in streaming mode
it results in a noop change to SVCR, if we are in streaming mode we will
exit as desired.

No functional change.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230110-arm64-sme-syscall-smstop-v1-1-ac94235fd810@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>