clippy: readonly_write_lock (anza-xyz#1336)

* clippy: readonly_write_lock

* add some comment

* update comments

ledger/src/blockstore.rs

@@ -11016,6 +11016,10 @@ pub mod tests {
                             let slot = shreds[0].slot();
                             // Grab this lock to block `get_slot_entries` before it fetches completed datasets
                             // and then mark the slot as dead, but full, by inserting carefully crafted shreds.
+
+                            #[allow(clippy::readonly_write_lock)]
+                            // Possible clippy bug, the lock is unused so clippy shouldn't care
+                            // about read vs. write lock
                             let _lowest_cleanup_slot =
                                 blockstore.lowest_cleanup_slot.write().unwrap();
                             blockstore.insert_shreds(shreds, None, false).unwrap();