kernel - Add vm.read_shortcut_enable

kernel - Add vm.read_shortcut_enable

* Add vm.read_shortcut_enable (disabled by default for now).  Set to 1 to
  enable this feature.

  This enables a helper function which HAMMER1 now uses to short-cut read()
  operations on files.  This feature only works on x86-64.

* When enabled this feature allows file read() requests to be satisfied
  directly from the VM page cache using lwbuf's, completely bypassing the
  buffer cache and also bypassing most of the VFS's VOP_READ code.

  The result is an approximate doubling of read() performance in cases
  where the buffer cache is too small to fit the hot data set, but the VM
  page cache is not.

  This feature is able to avoid the buffer cache and thus prevent buffer
  cycling within it which, due to the constant installation and
  deinstallation of pages in KVM cause a great deal of SMP page table
  page invalidations.