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.