bcachefs: Fix extent_sort_fix_overlapping()

bcachefs: Fix extent_sort_fix_overlapping()

Recently the extent update path started emmiting 0 size whiteouts on
extent overwrite, as part of transitioning to moving extent handling
out of the core btree code.

Unfortunately, this broke the old code path that handles overlapping
extents when reading in btree nodes - it relies on sorting incomming
extents by start position, but the 0 size whiteouts broke that ordering.
Skipping over them before the main algorithm sees them fixes this.

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