Kernel - Fix issues with vnode_pager_setsize() (effects NFS only)

Kernel - Fix issues with vnode_pager_setsize() (effects NFS only)

* When truncating a file to non-zero length, if the filesystem calls
  vnode_pager_setsize() with a truncation offset that is not a
  multiple of DEV_BSIZE vnode_pager_setsize() will improperly clear
  the dirty bit for the overlapping DEV_BSIZE'd block in the VM page
  straddling the truncation point.

  This only effects NFS.  The fsx filesystem test was failing due to
  a lost dirty page after a truncation.

Reported-by: Antonio Huete Jimenez <tuxillo@quantumachine.net>