bch2_btree_iter_traverse() is supposed to ensure we have the correct
type of lock - it was downgrading if necessary, but if we entered with a
read lock it wasn't upgrading to an intent lock, oops.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>