core: riscv: Simplify SP setup in setup_unwind_user_mode()

The parameter "regs" is the stack pointer which is allocated to store
system call registers when calling thread_scall_handler(). Thus, we can
simply get the original stack pointer by "regs + 1" equation, and use it
to exit user mode.

The code is referenced from ARM's setup_unwind_user_mode().

Signed-off-by: Alvin Chang <alvinga@andestech.com>

core/arch/riscv/kernel/thread_arch.c

@@ -100,7 +100,13 @@ static void setup_unwind_user_mode(struct thread_scall_regs *regs)
 {
 	regs->ra = (uintptr_t)thread_unwind_user_mode;
 	regs->status = xstatus_for_xret(true, PRV_S);
-	regs->sp = thread_get_saved_thread_sp();
+	/*
+	 * We are going to exit user mode because we handled an errored scall.
+	 * The stack pointer must be set as the original value it had before
+	 * allocating space of scall "regs" and calling thread_scall_handler().
+	 * Thus, we can simply set stack pointer as (regs + 1) value.
+	 */
+	regs->sp = (unsigned long)(regs + 1);
 }
 
 static void thread_unhandled_trap(unsigned long cause __unused,