This improves the bkey_format calculation when splitting btree nodes.
Previously, we'd use a format calculated for the original node for the
lower of the two new nodes.

This was particularly bad on sequential insertions, where we iteratively
split the last btree node, whos format has to include KEY_MAX.

Now, we calculate formats precisely for the keys the two new nodes will
contain. This also should make splitting a bit more efficient, since
we're only copying keys once (from the original node to the new node,
instead of new node, replacement node, then upper split).
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>