SUNRPC: Fix rpcgss_context trace event acceptor field

The rpcgss_context trace event acceptor field is a dynamically sized
string that records the "data" parameter. But this parameter is also
dependent on the "len" field to determine the size of the data.

It needs to use __string_len() helper macro where the length can be passed
in. It also incorrectly uses strncpy() to save it instead of
__assign_str(). As these macros can change, it is not wise to open code
them in trace events.

As of commit c759e609 ("tracing: Remove __assign_str_len()"),
__assign_str() can be used for both __string() and __string_len() fields.
Before that commit, __assign_str_len() is required to be used. This needs
to be noted for backporting. (In actuality, commit c1fa617c ("tracing:
Rework __assign_str() and __string() to not duplicate getting the string")
is the commit that makes __string_str_len() obsolete).

Cc: stable@vger.kernel.org
Fixes: 0c77668d ("SUNRPC: Introduce trace points in rpc_auth_gss.ko")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/trace/events/rpcgss.h

@@ -609,7 +609,7 @@ TRACE_EVENT(rpcgss_context,
 		__field(unsigned int, timeout)
 		__field(u32, window_size)
 		__field(int, len)
-		__string(acceptor, data)
+		__string_len(acceptor, data, len)
 	),
 
 	TP_fast_assign(
@@ -618,7 +618,7 @@ TRACE_EVENT(rpcgss_context,
 		__entry->timeout = timeout;
 		__entry->window_size = window_size;
 		__entry->len = len;
-		strncpy(__get_str(acceptor), data, len);
+		__assign_str(acceptor, data);
 	),
 
 	TP_printk("win_size=%u expiry=%lu now=%lu timeout=%u acceptor=%.*s",