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).