vm_fault: lookup page ranges for copy_entry

[dougm]

Summary

To reduce the number of searches of the radix trie in vm_fault_copy_entry(), attempt to read a range of pages, rather than a single page at a time.

Test Plan

The only way I have to test this is by running the mlockall.sh stress test, and testing with counters to count calls and cycles spent in vm_fault_copy_entry() indicates that this reduces performance slightly.
Before:
debug.counters.fault_copy_cycles: 66530662
debug.counters.fault_copy_calls: 90

After:
debug.counters.fault_copy_cycles: 68282538
debug.counters.fault_copy_calls: 90

[dougm]

Avoid using range-storing arrays too large for the kernel stack.