linux-user/mips/cpu_loop: silence the compiler warnings

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
linux-user/mips/cpu_loop.c: In function ‘cpu_loop’:
linux-user/mips/cpu_loop.c:104:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
  104 |                     if ((ret = get_user_ual(arg8, sp_reg + 28)) != 0) {
      |                        ^
linux-user/mips/cpu_loop.c:107:17: note: here
  107 |                 case 7:
      |                 ^~~~
linux-user/mips/cpu_loop.c:108:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
  108 |                     if ((ret = get_user_ual(arg7, sp_reg + 24)) != 0) {
      |                        ^
linux-user/mips/cpu_loop.c:111:17: note: here
  111 |                 case 6:
      |                 ^~~~
linux-user/mips/cpu_loop.c:112:24: warning: this statement may fall through [-Wimplicit-fallthrough=]
  112 |                     if ((ret = get_user_ual(arg6, sp_reg + 20)) != 0) {
      |                        ^
linux-user/mips/cpu_loop.c:115:17: note: here
  115 |                 case 5:
      |                 ^~~~

Add the corresponding "fall through" comment to fix it.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20201030004046.2191790-5-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/mips/cpu_loop.c

@@ -104,18 +104,22 @@ void cpu_loop(CPUMIPSState *env)
                     if ((ret = get_user_ual(arg8, sp_reg + 28)) != 0) {
                         goto done_syscall;
                     }
+                    /* fall through */
                 case 7:
                     if ((ret = get_user_ual(arg7, sp_reg + 24)) != 0) {
                         goto done_syscall;
                     }
+                    /* fall through */
                 case 6:
                     if ((ret = get_user_ual(arg6, sp_reg + 20)) != 0) {
                         goto done_syscall;
                     }
+                    /* fall through */
                 case 5:
                     if ((ret = get_user_ual(arg5, sp_reg + 16)) != 0) {
                         goto done_syscall;
                     }
+                    /* fall through */
                 default:
                     break;
                 }