Kernel - more NFS fixes, more dirty bit fixes, remove vfs_bio_set_validclean()

Kernel - more NFS fixes, more dirty bit fixes, remove vfs_bio_set_validclean()

* Remove vfs_bio_set_validclean().  It is no longer needed.

* General getpages operations must clear dirty bits non-inclusive of
  the end of the range.  A read which partially overlaps dirty VM
  pages shouldn't happen in the first place but if it were to happen
  we don't want to lose the dirty status on the DEV_BSIZE'd chunk
  straddling the end of the read.

* General truncation support.  Replace previous fix with a call to
  a new inline, vm_page_clear_dirty_beg_nonincl().  Similar to
  the getpages() issue, we do not want to lose the dirty status
  on the DEV_BSIZE'd chunk straddling the beginning of a truncation.

  (side note: Only effecs NFS as all other filesystems DEV_BSIZE-align
  their operations, but a good general fix in anycase).