SUNRPC: fix ternary sign expansion bug in tracing

This code is supposed to pass negative "err" values for tracing but it
passes positive values instead.  The problem is that the
trace_svcsock_tcp_send() function takes a long but "err" is an int and
"sent" is a u32.  The negative is first type promoted to u32 so it
becomes a high positive then it is promoted to long and it stays
positive.

Fix this by casting "err" directly to long.

Fixes: 998024de

 ("SUNRPC: Add more svcsock tracepoints")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>