KVM: set_memory_region: Disallow changing read-only attribute later

As Xiao pointed out, there are a few problems with it:
 - kvm_arch_commit_memory_region() write protects the memory slot only
   for GET_DIRTY_LOG when modifying the flags.
 - FNAME(sync_page) uses the old spte value to set a new one without
   checking KVM_MEM_READONLY flag.

Since we flush all shadow pages when creating a new slot, the simplest
fix is to disallow such problematic flag changes: this is safe because
no one is doing such things.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -874,12 +874,12 @@ It is recommended that the lower 21 bits of guest_phys_addr and userspace_addr
 be identical.  This allows large pages in the guest to be backed by large
 pages in the host.
 
-The flags field supports two flag, KVM_MEM_LOG_DIRTY_PAGES, which instructs
-kvm to keep track of writes to memory within the slot.  See KVM_GET_DIRTY_LOG
-ioctl.  The KVM_CAP_READONLY_MEM capability indicates the availability of the
-KVM_MEM_READONLY flag.  When this flag is set for a memory region, KVM only
-allows read accesses.  Writes will be posted to userspace as KVM_EXIT_MMIO
-exits.
+The flags field supports two flags: KVM_MEM_LOG_DIRTY_PAGES and
+KVM_MEM_READONLY.  The former can be set to instruct KVM to keep track of
+writes to memory within the slot.  See KVM_GET_DIRTY_LOG ioctl to know how to
+use it.  The latter can be set, if KVM_CAP_READONLY_MEM capability allows it,
+to make a new slot read-only.  In this case, writes to this memory will be
+posted to userspace as KVM_EXIT_MMIO exits.
 
 When the KVM_CAP_SYNC_MMU capability is available, changes in the backing of
 the memory region are automatically reflected into the guest.  For example, an