linux-user: Don't return uninitialized value for atomic_barrier syscall

QEMU's implementation of the m68k atomic_barrier syscall, like the kernel's,
is just a no-op. However we still need to return a result code from it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/syscall.c

@@ -9148,6 +9148,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
     case TARGET_NR_atomic_barrier:
     {
         /* Like the kernel implementation and the qemu arm barrier, no-op this? */
+        ret = 0;
         break;
     }
 #endif