bcachefs: Fix range in bch2_lookup_indirect_extent() error path

bcachefs: Fix range in bch2_lookup_indirect_extent() error path

Before calling bch2_indirect_extent_missing_error(), we have to
calculate the missing range, which is the intersection of the reflink
pointer and the non-indirect-extent we found.

The calculation didn't take into account that the returned extent may
span the iter position, leading to an infinite loop when we
(unnecessarily) resized the extent we were returning to one that didn't
extend past the offset we were looking up.

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