xfs: always set rvalp in xfs_dir2_node_trim_free

xfs_dir2_node_trim_free can return with setting the rvalp argument
pointer.  Initialize it to 0 at the beginning of the function and
only update it to 1 if we succeeded trimming a freespace block.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>