bitkeeper revision 1.1236.32.14 (423eb7a0HqJL37tAErMbIXIQw6Q3Jg)

bitkeeper revision 1.1236.32.14 (423eb7a0HqJL37tAErMbIXIQw6Q3Jg)

Added prediction of where to find the last writable PTE for a given page;
greatly speeds up promotion of a page to be used as a page table.

Removed some broken concepts of write protecting PDEs and higher level
entries.  To write protect a page, all we need to do is write protect all
L1 entries that point at it.

Fixed a bug with translated IO pages; gotta check that MFNs are really backed
by RAM before we go looking in the frame_table for them...

Signed-off-by: michael.fetterman@cl.cam.ac.uk