Btrfs: fix csum search offset/length calculation in log tree

We were setting the csums search offset and length to the right values if
the extent is compressed, but later on right before doing the csums lookup
we were overriding these two parameters regardless of compression being
set or not for the extent.
Signed-off-by: NFilipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

fs/btrfs/tree-log.c

@@ -3421,11 +3421,6 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 	if (skip_csum)
 		return 0;
 
-	if (em->compress_type) {
-		csum_offset = 0;
-		csum_len = block_len;
-	}
-
 	/*
 	 * First check and see if our csums are on our outstanding ordered
 	 * extents.
@@ -3509,8 +3504,13 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 	if (!mod_len || ret)
 		return ret;
 
-	csum_offset = mod_start - em->start;
-	csum_len = mod_len;
+	if (em->compress_type) {
+		csum_offset = 0;
+		csum_len = block_len;
+	} else {
+		csum_offset = mod_start - em->start;
+		csum_len = mod_len;
+	}
 
 	/* block start is already adjusted for the file extent offset. */
 	ret = btrfs_lookup_csums_range(log->fs_info->csum_root,