Btrfs: Hash in the offset and owner for file extent backref keys

This makes searches for backrefs and backref insertion much more efficient
when there are many backrefs for a single extent
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/extent-tree.c

@@ -397,13 +397,12 @@ static u64 hash_extent_ref(u64 root_objectid, u64 ref_generation,
 	high_crc = crc32c(high_crc, &lenum, sizeof(lenum));
 	lenum = cpu_to_le64(ref_generation);
 	low_crc = crc32c(low_crc, &lenum, sizeof(lenum));
-
-#if 0
-	lenum = cpu_to_le64(owner);
-	low_crc = crc32c(low_crc, &lenum, sizeof(lenum));
-	lenum = cpu_to_le64(owner_offset);
-	low_crc = crc32c(low_crc, &lenum, sizeof(lenum));
-#endif
+	if (owner >= BTRFS_FIRST_FREE_OBJECTID) {
+		lenum = cpu_to_le64(owner);
+		low_crc = crc32c(low_crc, &lenum, sizeof(lenum));
+		lenum = cpu_to_le64(owner_offset);
+		low_crc = crc32c(low_crc, &lenum, sizeof(lenum));
+	}
 	return ((u64)high_crc << 32) | (u64)low_crc;
 }