Btrfs: don't re-enter when allocating a chunk

If we start running low on metadata space we will try to allocate a chunk,
which could then try to allocate a chunk to add the device entry.  The thing
is we allocate a chunk before we try really hard to make the allocation, so
we should be able to find space for the device entry.  Add a flag to the
trans handle so we know we're currently allocating a chunk so we can just
bail out if we try to allocate another chunk.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/transaction.c

@@ -385,6 +385,7 @@ again:
 	h->qgroup_reserved = qgroup_reserved;
 	h->delayed_ref_elem.seq = 0;
 	h->type = type;
+	h->allocating_chunk = false;
 	INIT_LIST_HEAD(&h->qgroup_ref_list);
 	INIT_LIST_HEAD(&h->new_bgs);