Lock vmobjlock then check u_flags & UVM_VNODE_VALID in uvn_attach

Lock vmobjlock then check u_flags & UVM_VNODE_VALID in uvn_attach

This is a continuation of this commit:
"Always acquire the `vmobjlock' before incrementing an object's reference."

Unfortuantely this created a race found by syzkaller manifesting as:

panic: kernel diagnostic assertion "uvn->u_obj.uo_refs == 0" failed:
file "sys/uvm/uvm_vnode.c", line 234

ok mpi@

Reported-by: syzbot+dd2d2684ad2818c927da@syzkaller.appspotmail.com