xfs: delayed block allocation

xfs: delayed block allocation

There is no need to assign a zone util we submit the bio using it.
Assigning a zone earlier and incrementing the write pointer just means
we us up zone active resources longer than required.  So instead of
allocating the block in the iomap_begin and map_blocks methods, just
provide stub iomaps there, and then allocate the blocks just before
submitting the bios, which also fits in really nicely with the flow
to split the bios to the hardware limits.

Based on a really old recommendation from Damien.

Signed-off-by: Christoph Hellwig <hch@lst.de>