btrfs: fix return value check of btrfs_start_transaction()

The error check of btrfs_start_transaction() is added, and the mistake
of the error check on several places is corrected.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 6d66e5c..a4bbb85 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -3112,6 +3112,7 @@
 	BUG_ON(!path);
 
 	trans = btrfs_start_transaction(fs_info->tree_root, 0);
+	BUG_ON(IS_ERR(trans));
 
 	wc.trans = trans;
 	wc.pin = 1;