Btrfs: fix compiler warnings on 32 bit systems

The enospc tracing code added some interesting uses of
u64 pointer casts.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 016977b..04b77e3 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -327,7 +327,8 @@
 
 	if (num_bytes) {
 		trace_btrfs_space_reservation(root->fs_info, "transaction",
-					      (u64)h, num_bytes, 1);
+					      (u64)(unsigned long)h,
+					      num_bytes, 1);
 		h->block_rsv = &root->fs_info->trans_block_rsv;
 		h->bytes_reserved = num_bytes;
 	}