Correct an error in the previous revision that could lead to a panic:

Correct an error in the previous revision that could lead to a panic:
Found mapped cache page.  Specifically, if cnt.v_free_count dips below
cnt.v_free_reserved after p_start has been set to a non-NULL value,
then vm_map_pmap_enter() would break out of the loop and incorrectly
call pmap_enter_object() for the remaining address range.  To correct
this error, this revision truncates the address range so that
pmap_enter_object() will not map any cache pages.

In collaboration with: tegge@
Reported by: kris@