Introduce a new pmap function, pmap_extract_and_hold(). This function

Introduce a new pmap function, pmap_extract_and_hold().  This function
atomically extracts and holds the physical page that is associated with the
given pmap and virtual address.  Such a function is needed to make the
memory mapping optimizations used by, for example, pipes and raw disk I/O
MP-safe.

Reviewed by:	tegge