s3/loadparm: allocate a fresh sDefault object per lp_ctx

s3/loadparm: allocate a fresh sDefault object per lp_ctx

This is in preperation of preventing direct access to sDefault in all
places that currently modify it.

As currently s3/loadparm is afaict not accessing lp_ctx->sDefault, but
changes sDefault indirectly through lp_parm_ptr() this change is just a
safety measure to prevent future breakage.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=13051

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>