Remove unnecessary mmap in cpg

Code for zero-copy in cpg does following mmaps:
- Mmap anonymous, private memory to some address (-> malloc)
- Mmap shared memory of fd to address returned by first mmap
  (effectively shadows first mapping)

This is not necessary and only one mapping is needed.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Steven Dake <sdake@redhat.com>
Reviewed-by: Fabio M. Di Nitto <fdinitto@redhat.com>