bcachefs: Fix btree node split after merge operations

bcachefs: Fix btree node split after merge operations

A btree node merge operation deletes a key in the parent node; if when
inserting into the parent node we split the parent node, we can end up
with a whiteout in the parent node that we don't want.

The existing code drops them before doing the split, because they can
screw up picking the pivot, but we forgot about the unwritten writeouts
area - that needs to be cleared out too.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>