Use smaller 512 byte messages for portmapper messages

Portmapper messages are short and do not occupy more than 512 bytes.
Lower portmapper message size to 512 bytes. This change significantly
reduces the amount of memory needed when trying to establish a large
number of connections simultaneously. The old value is based on page
size.

Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>