arm64/fpsimd: Don't flush SME register hardware state along with thread

We recently changed the fpsimd thread flush to flush the physical SME
state as well as the thread state for the current thread.  Unfortunately
this leads to intermittent corruption in interaction with the lazy
FPSIMD register switching.  When under heavy load such as can be
triggered by the startup phase of fp-stress it is possible that the
current thread may not be scheduled prior to returning to userspace, and
indeed we may end up returning to the last thread that was scheduled on
the PE without ever exiting the kernel to any other task.  If that
happens then we will not reload the register state from memory, leading
to loss of any SME register state.

Since this was purely an attempt to defensively close off potential
problems revert the change.

Fixes: af3215f ("arm64/fpsimd: Exit streaming mode when flushing tasks")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230724-arm64-dont-flush-smstate-v1-1-9a8b637ace6c@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/fpsimd.c

@@ -1666,7 +1666,6 @@ void fpsimd_flush_thread(void)
 
 		fpsimd_flush_thread_vl(ARM64_VEC_SME);
 		current->thread.svcr = 0;
-		sme_smstop();
 	}
 
 	current->thread.fp_type = FP_STATE_FPSIMD;