arm64: fpsimd: Fix copying of FP state from signal frame into task struct

Commit 9de52a75 ("arm64: fpsimd: Fix failure to restore FPSIMD
state after signals") fixed an issue reported in our FPSIMD signal
restore code but inadvertently introduced another issue which tends to
manifest as random SEGVs in userspace.

The problem is that when we copy the struct fpsimd_state from the kernel
stack (populated from the signal frame) into the struct held in the
current thread_struct, we blindly copy uninitialised stack into the
"cpu" field, which means that context-switching of the FP registers is
no longer reliable.

This patch fixes the problem by copying only the user_fpsimd member of
struct fpsimd_state. We should really rework the function prototypes
to take struct user_fpsimd_state * instead, but let's just get this
fixed for now.

Cc: Dave Martin <Dave.Martin@arm.com>
Fixes: 9de52a75 ("arm64: fpsimd: Fix failure to restore FPSIMD state after signals")
Reported-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/fpsimd.c

@@ -1043,7 +1043,7 @@ void fpsimd_update_current_state(struct fpsimd_state *state)
 
 	local_bh_disable();
 
-	current->thread.fpsimd_state = *state;
+	current->thread.fpsimd_state.user_fpsimd = state->user_fpsimd;
 	if (system_supports_sve() && test_thread_flag(TIF_SVE))
 		fpsimd_to_sve(current);