vnode_pager_haspage() could return TRUE but leave *before and *after

vnode_pager_haspage() could return TRUE but leave *before and *after
uninitialized, causing vm_fault's burst pagein feature to panic the system.

vm_fault was almost never using its burst pagein feature due to incorrect
test logic.  The burst pagein code itself was also seriously buggy, so it
is fortunate the test logic was broken.

Rewrite the broken test logic and fix the bugs in the burst pagein code.

Add a new sysctl vm.burst_fault, defaulting to 0 (disabled).  The default
will be changed to 1 in a week or two.