pidl: calculate subcontext_size only once per pull

pidl: calculate subcontext_size only once per pull

For security_ace_coda in security.idl, the sub-context size is
involves a slightly non-trivial function call which returns a constant
value.

In all other cases, a constant expression is used, and this makes
no difference.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=15574
(cherry picked from commit 9811762775b28e16035afb2c319b55c4bf3699d3)