bcachefs: BCH_FEATURE_new_extent_overwrite is now required

bcachefs: BCH_FEATURE_new_extent_overwrite is now required

The patch "bcachefs: Move extent overwrite handling out of core btree
code" should have been flipping on this feature bit; extent btree nodes
in the old format have to be rewritten before we can insert into them
with the new extent update path. Not turning on this feature bit was
causing us to go into an infinite loop where we keep rewriting btree
nodes over and over.

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