kmod/toa: fix is_ro_addr()

[yongjianchn]

#484 by @goecho was trying to fix crash problem on some kernel versions. The new function is_ro_addr() is used to check the address is ro or rw.

but the condition seems weired:

if (pte->pte &~ _PAGE_RW) {
  ro_enable = 1;
}

pte->pte &~ _PAGE_RW means there are some other flags being set, no matter RW flags is set or not.

[ywc689]

It seems good to me. But I wonder why a lot of others also do as this? Did you test your patch? Did it work OK?

https://stackoverflow.com/questions/2103315/linux-kernel-system-call-hooking-example
https://lwn.net/Articles/584198/

[yongjianchn]

With #484 , I got crash on linux-4.9 (build by myself).
Then I found the wrong if condition, which "thinks" the related addrs is RO, and it did the set_rw() and set_ro()，then seconds later, my kernel crashed.
Actually, the addrs is RW, I changed the if condition (like this pull request), everything works fine.

I also test the latest CentOS 7, which also works fine.

[yongjianchn]

I changed base branch from master to devel.

Test on:

• the latest CentOS 7 (kernel-3.10.0-1127.el7.x86_64)
• the latest CentOS 8 (kernel-4.18.0-193.19.1.el8_2.x86_64)