crimson/os/seastore/cached_extent: add comments to elaborate why

MUTATION_PENDING and under-io extents are "stable"

Signed-off-by: Xuehan Xu <[email protected]>

src/crimson/os/seastore/cached_extent.h

@@ -426,6 +426,9 @@ class CachedExtent
   /// Returns true if extent is stable and shared among transactions
   bool is_stable() const {
     return is_stable_written() ||
+	   // MUTATION_PENDING and under-io extents are to-be-stable extents,
+	   // for the sake of caveats that checks the correctness of extents
+	   // states, we consider them stable.
            (is_mutation_pending() &&
             is_pending_io());
   }