kernel - Add burst reads back into swap_pager

kernel - Add burst reads back into swap_pager

* Burst reading from swap was disabled due to recent VM pager changes
  which changes vm_pager_getpages() from specifying multiple-pages to
  specifying just one-page.

* Add burst reading back into pretty much the only pager left (other
  then devfs_pager and phys_pager which are special cases) which
  doesn't use the buffer cache.

* The code may still be a bit unstable so it is diabled by default.
  Add a new sysctl vm.swap_burst_read (defaults to off) which can
  be used to enable burst reading from swap.