util: Use linear parent to (r)allocated extra nodes

util: Use linear parent to (r)allocated extra nodes

Instead of making all the nodes children of the same original
ralloc_ctx, use that context just for the first node (which
is the linear parent).  The remaining nodes are (r)allocated
using the first one.

With that change, to free/steal, it is sufficient to apply the
operation to the first node, which uses the underlying linked
list between the nodes at ralloc level.  And we can remove
the 'next' pointer in linear_header, now unused.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25280>