arch/risc-v: Correct FPU register save area in riscv_copystate

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

arch/risc-v/src/common/riscv_copystate.c

@@ -55,10 +55,6 @@ void riscv_copystate(uintptr_t *dest, uintptr_t *src)
 {
   int i;
 
-#ifdef CONFIG_ARCH_FPU
-  uintptr_t *regs = dest;
-#endif
-
   /* In the RISC-V model, the state is copied from the stack to the TCB,
    * but only a reference is passed to get the state from the TCB.  So the
    * following check avoids copying the TCB save area onto itself:
@@ -79,7 +75,7 @@ void riscv_copystate(uintptr_t *dest, uintptr_t *src)
        */
 
 #ifdef CONFIG_ARCH_FPU
-      riscv_savefpu(regs);
+      riscv_savefpu(dest);
 #endif
     }
 }