bcachefs: Fix a null ptr deref during journal replay

bcachefs: Fix a null ptr deref during journal replay

We were calling bch2_extent_can_insert() incorrectly; it should only be
called when the extents-to-keys pass is running because that's when we
could be splitting a compressed extent. Calling bch2_extent_can_insert()
without passing in a disk reservation was causing a null ptr deref.

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