tools/net/ynl: Fix c codegen for array-nest

ynl-gen-c generates e.g. 'calloc(mcast_groups, sizeof(*dst->mcast_groups))'
for array-nest attrs when it should be 'n_mcast_groups'.

Add a 'n_' prefix in the generated code for array-nests.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20240306231046.97158-4-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>