arm64/fpsimd: Exit streaming mode when flushing tasks

Ensure there is no path where we might attempt to save SME state after we
flush a task by updating the SVCR register state as well as updating our
in memory state. I haven't seen a specific case where this is happening or
seen a path where it might happen but for the cost of a single low overhead
instruction it seems sensible to close the potential gap.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230607-arm64-flush-svcr-v2-1-827306001841@kernel.org


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