Replace KASSERT in uvm_fault_unwire_locked() with code that handles the

Replace KASSERT in uvm_fault_unwire_locked() with code that handles the
case where not all pages are wired.  The KASSERT can be triggered in
multi-threaded applications when a thread calling munmap(2) races another
thread that invokes sysctl(2).  Properly written code shouldn't do this,
but making the kernel crash in this case is a bit harsh.

ok gezdo@, deraadt@

Fixes:
Reported-by: syzbot+e8310909e2910c9cca08@syzkaller.appspotmail.com