block: qcow: Cache cluster_size in per queue structs

Cache the immutable cluster_size value at construction time in
QcowAsync, QcowSync, and Qcow2Backing. This avoids repeated RwLock
read acquisitions on the hot write and deallocation paths.

Replace QcowMetadata::cluster_offset() calls with inline bitmask
operations using the cached cluster_size. Remove the now unused
cluster_offset() method from QcowMetadata.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/qcow/metadata.rs

@@ -333,11 +333,6 @@ impl QcowMetadata {
     pub fn cluster_size(&self) -> u64 {
         self.inner.read().unwrap().raw_file.cluster_size()
     }
-
-    /// Returns the intra cluster byte offset for a given guest address.
-    pub fn cluster_offset(&self, address: u64) -> u64 {
-        self.inner.read().unwrap().raw_file.cluster_offset(address)
-    }
 }
 
 impl QcowState {