nfs-utils: Provide type-checked version of svc_getcaller()

TI-RPC's version of the svc_getcaller() macro points to a sockaddr_in6,
not a sockaddr_in, though for AF_INET callers, an AF_INET address
resides there.  To squelch compiler warnings when the TI-RPC version of
the svc_req structure is used, add inline helpers with appropriate
type casting.

Note that tcp_wrappers support only AF_INET addresses for now.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>