vm: add unlocked page lookup before trying vm_fault_soft_fast

Shaves a read lock + tryupgrade trip most of the time.

Stats from doing a kernel build (counters not present in the tree):
vm.fault_soft_fast_ok: 262653
vm.fault_soft_fast_failed_other: 41
vm.fault_soft_fast_failed_no_page: 39595772
vm.fault_soft_fast_failed_page_busy: 1929
vm.fault_soft_fast_failed_page_invalid: 22183

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D39268