Rearrange code in lafs_cluster_allocate.

Rearrange code in lafs_cluster_allocate.

There are two loops in lafs_cluster_allocate that I want
to combine into one.
Need to get some stuff out of the way first.

EmptyIndex handling can move up to the other special-case
handling - just change writeback_done to iounlock_block.

Converting the iolock to writeback and getting a cluster reference
can also move up easily.

Signed-off-by: NeilBrown <neilb@suse.de>