Prep work for the patch to update existing stripes with new data blocks.
This moves allocating new stripes into ec.c, and also sets up the data
structures so that we can handly only allocating some of the blocks in a
stripe.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>