pnfsblock: alloc short extent before submit bio

As discussed earlier, it is better for block client to allocate memory for
tracking extents state before submitting bio. So the patch does it by allocating
a short_extent for every INVALID extent touched by write pagelist and for
every zeroing page we created, saving them in layout header. Then in end_io we
can just use them to create commit list items and avoid memory allocation there.

Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>