xfs: remove xfs_buf_target_name

The calling convention that returns a pointer to a static buffer is
fairly nasty, so just opencode it in the only caller that is left.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>