This improves io_opts() and makes it a non-inline function - it's big
enough that it probably shouldn't be.

Also, bch_io_opts no longer needs fields for whether options are
defined, so we can slim it down a bit.

We'd like to stop passing around the full bch_io_opts, but that'll be
tricky because of bch2_rebalance_add_key().
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>