Fixes a pet peeve: the identifier "flushpage" implies "flush the page
to disk".  Which is very much not what the flushpage functions actually
do.

The patch renames block_flushpage and the flushpage
address_space_operation to "invalidatepage".

It also fixes a buglet in invalidate_this_page2(), which was calling
block_flushpage() directly - it needs to call do_flushpage() (now
do_invalidatepage()) so that the filesystem's ->flushpage (now
->invalidatepage) a_op gets a chance to relinquish any interest which
it has in the page's buffers.