bcachefs: Always emit new extents on partial overwrite

This is prep work for snapshots: the algorithm in
bch2_extent_sort_fix_overlapping() will break when we have multiple
overlapping extents in unrelated snapshots - but, we'll be able to make
extents work like regular keys and use bch2_key_sort_fix_overlapping()
for extent btree nodes if we make a couple changes - the main one being
to always emit new extents when we partially overwrite an existing
(written) extent.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>