arm64/ptrace: Flush FP state when setting ZT0

When setting ZT0 via ptrace we do not currently force a reload of the
floating point register state from memory, do that to ensure that the newly
set value gets loaded into the registers on next task execution.

The function was templated off the function for FPSIMD which due to our
providing the option of embedding a FPSIMD regset within the SVE regset
does not directly include the flush.

Fixes: f90b529 ("arm64/sme: Implement ZT0 ptrace support")
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20230803-arm64-fix-ptrace-zt0-flush-v1-1-72e854eaf96e@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/ptrace.c

@@ -1180,6 +1180,8 @@ static int zt_set(struct task_struct *target,
 	if (ret == 0)
 		target->thread.svcr |= SVCR_ZA_MASK;
 
+	fpsimd_flush_task_state(target);
+
 	return ret;
 }