We don't need to pass the number of nodes required to
bch2_btree_update_start, just whether we're doing a split at @level.
This is prep work for a fix to our usage of six lock's percpu mode,
which is going to require us to count up and allocate interior nodes and
leaf nodes seperately.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>