Fix the deadlock between uvn_io() and uvn_flush() by restarting the fault.

Fix the deadlock between uvn_io() and uvn_flush() by restarting the fault.

Do not allow a faulting thread to sleep on a contended vnode lock to prevent
lock ordering issues with upcoming per-uobj lock.

Also reduce the sleep value for VM_PAGER_AGAIN from 1sec to 5nsec to not add
visible slowdown when starting a multi-threaded application with threads that
fault on the same vnode (chromium, firefox, etc).

Tested by anton@, tb@, robert@ and gnezdo@

ok anton@, tb@

Reported-by: syzbot+e63407b35dff08dbee02@syzkaller.appspotmail.com