mmu: Fix page_ops initialization for anonymous VMAs

_flags is currently being used to determine which page_ops will be assigned to
the anonymous VMA, however, the variable flags received as a parameter is the
one that must be used for that purpose.

This problem was found while searching for the root of the general protection
faults that were often happening when running our test suite.
Bisect pointed to the commit 55693e5c
The GPF was probably happening due to code that should only proceed with
completely initialized anonymous VMAs.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Reviewed-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

core/mmu.cc

@@ -1141,7 +1141,7 @@ static map_anon_page page_ops_init;
 static map_page_ops *page_ops_noinitp = &page_ops_noinit, *page_ops_initp = &page_ops_init;
 
 anon_vma::anon_vma(addr_range range, unsigned perm, unsigned flags)
-    : vma(range, perm, flags, true, (_flags & mmap_uninitialized) ? page_ops_noinitp : page_ops_initp)
+    : vma(range, perm, flags, true, (flags & mmap_uninitialized) ? page_ops_noinitp : page_ops_initp)
 {
 }