SUNRPC: Fix general protection fault in trace_rpc_xdr_overflow()

The TP_fast_assign() section is careful enough not to dereference
xdr->rqst if it's NULL. The TP_STRUCT__entry section is not.

Fixes: 5582863f

 ("SUNRPC: Add XDR overflow trace event")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>