kernel - Support bdwrite() on tmpfs buffer cache buffers

kernel - Support bdwrite() on tmpfs buffer cache buffers

* tmpfs itself does not call bdwrite() when no swap space has been
  allocated but certain system side effects such as nvtruncbuf()
  when a file is truncated CAN cause bdwrite() to be called.

  This eventually causes the buffer to run through tmpfs's strategy
  function.

* Adjust the strategy function to handle the no-swap case by marking
  the underlying pages dirty and valid and completing the I/O.

* Also handle the case when swap exists but fills up.  If the
  swap_strategy() function fails with ENOMEM fall-back to again
  marking the pages dirty andvalid and completing the I/O.

  This allows tmpfs to fill up to the system's total memory+swap instead
  of just the system's swap.