bcachefs: Add bch_io_opts fields for indicating whether the opts came from the inode

This is going to be used in the bch_extent_rebalance improvements, which
propagate io_path options into the extent (important for rebalance,
which needs something present in the extent for transactionally tagging
them in the rebalance_work btree, and also for indirect extents).

By tracking in bch_extent_rebalance whether the option came from the
filesystem or the inode we can correctly handle options being changed on
indirect extents.

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