Make sure uvm_swap_get() always sleep and do not return and error.

Make sure uvm_swap_get() always sleep and do not return and error.

If no memory was immediately available to decrypt (bounce) a page from swap
an error was returned to the fault handler which would result in processes
dying when a lot of memory pressure was applied to a system.

Note that reading from swap is always done synchronously.

ok beck@, kettenis@