bcachefs: BCH_READ_data_update -> bch_read_bio.data_update

bcachefs: BCH_READ_data_update -> bch_read_bio.data_update

Read flags are codepath dependent and change as they're passed around,
while the fields in rbio._state are mostly fixed properties of that
particular object.

Losing track of BCH_READ_data_update would be bad, and previously it was
not obvious if it was always correctly set in the rbio, so this is a
safety cleanup.

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