block: qcow: Move compressed read decompression out of lock

Move decompression of compressed QCOW2 clusters out of the metadata
lock. Previously, reading a compressed cluster acquired a write lock
on metadata to perform in place decompression. Now, try_map_read
extracts the compressed layout (host offset, size) under a read lock
and returns it in the ClusterReadMapping::Compressed variant. Each
consumer (QcowSync, QcowAsync, Qcow2Backing, QcowFile) performs the
pread and decompression at the call site without holding any lock,
using the pread_alloc and decompress_cluster helpers.

Create the decoder once in QcowMetadata as Arc<dyn Decoder> and
share it via Arc::clone to QcowAsync, QcowSync, and Qcow2Backing
at construction time. This avoids per read RwLock acquisitions and
heap allocations. Add Send + Sync bounds to the Decoder trait.

This eliminates write lock contention on compressed reads, allowing
them to proceed concurrently with other read operations.

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

block/src/qcow/decoder.rs

@@ -19,7 +19,7 @@ pub enum Error {
 pub type Result<T> = std::result::Result<T, Error>;
 
 /// Generic trait for decoding zlib/zstd formats
-pub trait Decoder {
+pub trait Decoder: Send + Sync {
     fn decode(&self, input: &[u8], output: &mut [u8]) -> Result<usize>;
 }