bcachefs: Only run check_backpointers_to_extents in debug mode

bcachefs: Only run check_backpointers_to_extents in debug mode

The backpointers passes, check_backpointers_to_extents() and
check_extents_to_backpointers() are the most expensive fsck passes.

Now that we're running the same check and repair code when using a
backpointer at runtime (via bch2_backpointer_get_key()) that fsck does,
there's no reason fsck needs to - except to verify that the filesystem
really has no errors in debug mode.

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