block: qcow: Add SyncingHeader error variant for fsync operations

Replace generic WritingHeader error with specific SyncingHeader
error for header fsync operations. This provides more precise
error reporting when syncing QCOW2 header changes to disk fails.

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

block/src/qcow/mod.rs

@@ -112,6 +112,8 @@ pub enum Error {
     SettingRefcountRefcount(#[source] io::Error),
     #[error("Size too small for number of clusters")]
     SizeTooSmallForNumberOfClusters,
+    #[error("Failed to sync header")]
+    SyncingHeader(#[source] io::Error),
     #[error("L1 entry table too large: {0}")]
     TooManyL1Entries(u64),
     #[error("Ref count table too large: {0}")]
@@ -698,7 +700,7 @@ impl QcowHeader {
                 self.incompatible_features &= !IncompatFeatures::DIRTY.bits();
             }
             self.write_incompatible_features(file)?;
-            file.fsync().map_err(Error::WritingHeader)?;
+            file.fsync().map_err(Error::SyncingHeader)?;
         }
         Ok(())
     }
@@ -711,7 +713,7 @@ impl QcowHeader {
         if self.version == 3 {
             self.incompatible_features |= IncompatFeatures::CORRUPT.bits();
             self.write_incompatible_features(file)?;
-            file.fsync().map_err(Error::WritingHeader)?;
+            file.fsync().map_err(Error::SyncingHeader)?;
         }
         Ok(())
     }
@@ -734,7 +736,7 @@ impl QcowHeader {
             file.seek(SeekFrom::Start(AUTOCLEAR_FEATURES_OFFSET))
                 .map_err(Error::WritingHeader)?;
             u64::write_be(file, 0).map_err(Error::WritingHeader)?;
-            file.fsync().map_err(Error::WritingHeader)?;
+            file.fsync().map_err(Error::SyncingHeader)?;
         }
         Ok(())
     }