smb/client: avoid possible NULL dereference in cifs_free_subrequest()

Clang static checker (scan-build) warning:
	cifsglob.h:line 890, column 3
	Access to field 'ops' results in a dereference of a null pointer.

Commit 519be989 ("cifs: Add a tracepoint to track credits involved in
R/W requests") adds a check for 'rdata->server', and let clang throw this
warning about NULL dereference.

When 'rdata->credits.value != 0 && rdata->server == NULL' happens,
add_credits_and_wake_if() will call rdata->server->ops->add_credits().
This will cause NULL dereference problem. Add a check for 'rdata->server'
to avoid NULL dereference.

Cc: stable@vger.kernel.org
Fixes: 69c3c023 ("cifs: Implement netfslib hooks")
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Su Hui <suhui@nfschina.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/file.c

@@ -315,7 +315,7 @@ static void cifs_free_subrequest(struct netfs_io_subrequest *subreq)
 #endif
 	}
 
-	if (rdata->credits.value != 0)
+	if (rdata->credits.value != 0) {
 		trace_smb3_rw_credits(rdata->rreq->debug_id,
 				      rdata->subreq.debug_index,
 				      rdata->credits.value,
@@ -323,8 +323,12 @@ static void cifs_free_subrequest(struct netfs_io_subrequest *subreq)
 				      rdata->server ? rdata->server->in_flight : 0,
 				      -rdata->credits.value,
 				      cifs_trace_rw_credits_free_subreq);
+		if (rdata->server)
+			add_credits_and_wake_if(rdata->server, &rdata->credits, 0);
+		else
+			rdata->credits.value = 0;
+	}
 
-	add_credits_and_wake_if(rdata->server, &rdata->credits, 0);
 	if (rdata->have_xid)
 		free_xid(rdata->xid);
 }