bch2_extent_atomic_end counts the number of iterators requried for
marking overwrites - but journal replay never marks overwrites, so that
part was incorrect. And counting iterators for the key being inserted
should be unnecessary because we did that prior to the key being
inserted before it was first journalled.

This should fix an iterator overflow bug - the iterators for walking
overwrites were totally unneeded.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>