xen/xenbus: Fix a double free in xenbus_map_ring_pv()

When there is an error the caller frees "info->node" so the free here
will result in a double free.  We should just delete first kfree().

Fixes: 3848e4e0

 ("xen/xenbus: avoid large structs and arrays on the stack")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20200710113610.GA92345@mwanda
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>