bcachefs: Count iterators for reflink_p overwrites correctly

bcachefs: Count iterators for reflink_p overwrites correctly

In order to avoid trying to allocate too many btree iterators,
bch2_extent_atomic_end() needs to count how many iterators are going to
be needed for insertions and overwrites - but we weren't counting the
iterators for deleting a reflink_v when the refcount goes to 0.

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