Pages under writeback are not locked.  So it is possible (and quite
legal) for a page to be under readpage() while it is still under
writeback.  For a partially uptodate page with blocksize <
PAGE_CACHE_SIZE.

When this happens, the read and write I/O completion handlers get
confused over the shared BH_Async usage and the page ends up not
getting PG_writeback cleared.  Truncate gets stuck in D state.

The patch separates the read and write I/O completion state.

It also shuffles the buffer fields around.  Putting the
commonly-accessed b_state at offset zero shrinks the kernel by a few
hundred bytes because it can be accessed with indirect addressing, not
indirect+indexed.