riscv: avoid Imbalance in RAS

Inspired by[1], modify the code to remove the code of modifying ra to
avoid imbalance RAS (return address stack) which may lead to incorret
predictions on return.

Link: https://lore.kernel.org/linux-riscv/[email protected]/ [1]
Signed-off-by: Jisheng Zhang <[email protected]>
Reviewed-by: Cyril Bur <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: xhackerustc

arch/riscv/kernel/entry.S

@@ -326,8 +326,8 @@ SYM_CODE_START(ret_from_fork)
 	jalr s0
 1:
 	move a0, sp /* pt_regs */
-	la ra, ret_from_exception
-	tail syscall_exit_to_user_mode
+	call syscall_exit_to_user_mode
+	j ret_from_exception
 SYM_CODE_END(ret_from_fork)
 
 #ifdef CONFIG_IRQ_STACKS