vm_fault: add a rangelock to vm_map
and take the wlock in the range of (fault_address, fault_address +
PAGE_SIZE) around calls to vm_fault(). This would be needed later
when page to be mapped might be busied shared for some soft faults.
vm_fault: add helper vm_fault_can_cow_rename()
vm_fault: try to only share-busy page for soft faults
If the fault found a vaild page that is definitely not going to be
renamed for Cow, try to only sbusy the page. We do not need to validate
the page, and parallel faults on the same address are excluded by the
map' fltlock.