bcachefs: Fix implementation of KEY_TYPE_error

bcachefs: Fix implementation of KEY_TYPE_error

When force-removing a device, we were silently dropping extents that we
no longer had pointers for - we should have been switching them to
KEY_TYPE_error, so that reads for data that was lost return errors.

This patch adds the logic for switching a key to KEY_TYPE_error to
bch2_bkey_drop_ptr(), and improves the logic somewhat.

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