[RFC, PATCH] fix verify_write for 80386 support

[Manfred Spraul]

Real 80386 cpus have a special feature: they ignore the write-protected 
bit in the page tables if the kernel runs at ring 0. This feature breaks 
COW, the software must manually check the write protected bit before 
writing.

Right now the check happens only in access_ok(VERIFY_WRITE).
This is broken, because:
- some codepaths call access_ok(VERIFY_READ), and then write.
- kswapd might change the page permissions between access_ok and the 
actual write access.