Documentation/watchdog: check return value for magic close

A recent commit added a write to the watchdog test code for doing the "magic
close", but that caused a compile-time warning:

Documentation/watchdog/src/watchdog-test.c: In function ‘main’:
Documentation/watchdog/src/watchdog-test.c:94:5: warning: ignoring return value of ‘write’, declared with attribute warn_unused_result [-Wunused-result]

This changes the code to print a runtime warning if the write fails.

Fixes: 5a2d3de19602 ("Documentation/watchdog: add support for magic close to watchdog-test")
Signed-off-by: Arnd Bergmann <[e-mail]>
Acked-by: Timur Tabi <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

Documentation/watchdog/src/watchdog-test.c

···
  * Watchdog Driver Test Program
  */
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
···
 
 static void term(int sig)
 {
-    write(fd, &v, 1);
+    int ret = write(fd, &v, 1);
+
     close(fd);
-    printf("\nStopping watchdog ticks...\n");
+    if (ret < 0)
+	printf("\nStopping watchdog ticks failed (%d)...\n", errno);
+    else
+	printf("\nStopping watchdog ticks...\n");
     exit(0);
 }
 
···
 {
     int flags;
     unsigned int ping_rate = 1;
+    int ret;
 
     setbuf(stdout, NULL);
 
···
 	sleep(ping_rate);
     }
 end:
-    write(fd, &v, 1);
+    ret = write(fd, &v, 1);
+    if (ret < 0)
+	printf("Stopping watchdog ticks failed (%d)...\n", errno);
     close(fd);
     return 0;
 }