xfs: distinguish extra split from real ENOSPC from xfs_attr_node_try_addname

Just like xfs_attr3_leaf_split, xfs_attr_node_try_addname can return
-ENOSPC both for an actual failure to allocate a disk block, but also
to signal the caller to convert the format of the attr fork.  Use magic
1 to ask for the conversion here as well.

Note that unlike the similar issue in xfs_attr3_leaf_split, this one was
only found by code review.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>