bcachefs: Improve bch2_inode_opts_to_opts()

bcachefs: Improve bch2_inode_opts_to_opts()

It turns out the *_defined entries of bch_io_opts are only used in one
place - in the xattr get path - and there we immediately convert to a
bch_opts struct, which also has the *_defined entries.

This patch changes bch2_inode_opts_to_opts() to go directly from
bch_inode_unpacked to bch_opts, which is a minor simplification and will
also let us slim down struct bch_io_opts in another patch.

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