sys/cddl: Strip out 'extern vmem_t *zio_alloc_arena'

Does not seem to be used and just keeps build.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
cddl: Internalise VOP_GETATTR to __VOP_GETATTR to avoid conflict

REVIST

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>