SUNRPC: Fix error return value of svc_addr_len()

The svc_addr_len() helper function returns -EAFNOSUPPORT if it doesn't
recognize the address family of the passed-in socket address.  However,
the return type of this function is size_t, which means -EAFNOSUPPORT
is turned into a very large positive value in this case.

The check in svc_udp_recvfrom() to see if the return value is less
than zero therefore won't work at all.

Additionally, handle_connect_req() passes this value directly to
memset().  This could cause memset() to clobber a large chunk of memory
if svc_addr_len() has returned an error.  Currently the address family
of these addresses, however, is known to be supported long before
handle_connect_req() is called, so this isn't a real risk.

Change the error return value of svc_addr_len() to zero, which fits in
the range of size_t, and is safer to pass to memset() directly.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>