bcachefs: Fix lifetime issue in device iterator helpers

bch2_get_next_dev() and bch2_get_next_online_dev() iterate over devices,
dropping and taking refs as they go; we can't access the previous device
(for ca->dev_idx) after we've dropped our ref to it, unless we take
rcu_read_lock() first.

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