These two patches allow conversion of a written range of a file into
back into unwritten state. The first patch converts page cache
helper functions to use the correct range primitives as we need to
be able to toss (invalidate) pages only within the range specified,
not to to the end of the file as it currently uses.
The actual conversion also preallocates any holes in the range, so
it turns the entire range requested into allocated, unwritten
o removed stray debugging printks
o truncate_inode_pages_range doesn't handle partial tail pages, so
the range needs to be tossed appropriately.