kernel - TMPFS - Add infrastructure to main kernel to help support TMPFS

kernel - TMPFS - Add infrastructure to main kernel to help support TMPFS

* Add buwrite(), similar to bdwrite() except it fakes the write,
  marks the pages as valid and dirty, and returns them to the VM
  system leaving the buffer cache buffer clean.

  This is used by tmpfs in tmpfs_write() and allows the entire VM page
  cache to be used to cache dirty tmpfs data instead of just the
  buffer cache.

  Also add vm_page_ste_validdirty() to suppor buwrite().

* Implement MNTK_SG_MPSAFE for future use by tmpfs.

* Fix a bug in swap_strategy().  When the entire block being
  requested is sparse (has no swap assignments) the function
  was not properly biodone()'ing the original bio after
  zero-filling the space.