This solves internal fragmentation within stripes. We already have
copygc, which evacuates buckets that are partially or mostly empty, but
it's up to the ec code that manages stripes to deal with stripes that
have empty buckets in them.

This patch changes the path for creating new stripes to check if there's
existing stripes with empty buckets - and if so, update them with new
data buckets instead of creating new stripes.

TODO: improve the disk space accounting so that we can only use this
(more expensive path) when we have too much fragmentation in existing
stripes.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>