iomap: split bios to zone append limits in the submission handlers

iomap: split bios to zone append limits in the submission handlers

Provide helpers for file systems to split bios in the direct I/O and
writeback I/O submission handlers.  The split ioends are chained to
the parent ioend so that only the parent ioend originally generated
by the iomap layer will be processed after all the chained off children
have completed.  This is based on the block layer bio chaining that has
supported a similar mechanism for a long time.

This Follows btrfs' lead and don't try to build bios to hardware limits
for zone append commands, but instead build them as normal unconstrained
bios and split them to the hardware limits in the I/O submission handler.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>