Fix a portability issue in svc_dg_recv().

Size of 'sa_family_t' data type differs across platforms (2 bytes on Linux
x86_64 vs 1 byte on amd64 FreeBSD). Assigning a value to a variable of data type
sa_family_t thus requires an explicit cast to avoid the strict compile errors.

Signed-off-by: Sachin Bhamare <sbhamare@panasas.com>