Close two holes in the pmap code. The page table self mapping does not

Close two holes in the pmap code.  The page table self mapping does not
necessarily stay valid through a blocking condition, recheck it for every
page.  The 'next' pointer in the loop that runs through PV entries is not
safe when code blocks in the loop.  Add a generation count to detect
unexpected removals and restart the loop.  Add a kprintf() to print when
the condition is detected.

Crash-Reported-by: Nuno Antunes <bugs@crater.dragonflybsd.org>