DESC
remap_file_pages protection support: restrict permission testing
EDESC
From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

Yet to test. Currently we install a PTE when one is missing
irrispective of the fault type, and if the access type is prohibited we'll
get another fault and kill the process only then. With this, we check the
access type on the 1st fault.

We could also use this code for testing present PTE's, if the current
assumption (fault on present PTE's in VM_NONUNIFORM vma's means access violation)
proves problematic for architectures other than UML (which I already fixed),
but I hope it's not needed.
