kobject_uevent: remove warning in init_uevent_argv()

syzbot can trigger the WARN() in init_uevent_argv() which isn't the
nicest as the code does properly recover and handle the error.  So
change the WARN() call to pr_warn() and provide some more information on
what the buffer size that was needed.

Link: https://lore.kernel.org/r/20201107082206.GA19079@kroah.com
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: linux-kernel@vger.kernel.org
Reported-by: syzbot+92340f7b2b4789907fdb@syzkaller.appspotmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210405094852.1348499-1-gregkh@linuxfoundation.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/kobject_uevent.c

@@ -251,12 +251,13 @@ static int kobj_usermode_filter(struct kobject *kobj)
 
 static int init_uevent_argv(struct kobj_uevent_env *env, const char *subsystem)
 {
+	int buffer_size = sizeof(env->buf) - env->buflen;
 	int len;
 
-	len = strlcpy(&env->buf[env->buflen], subsystem,
-		      sizeof(env->buf) - env->buflen);
-	if (len >= (sizeof(env->buf) - env->buflen)) {
-		WARN(1, KERN_ERR "init_uevent_argv: buffer size too small\n");
+	len = strlcpy(&env->buf[env->buflen], subsystem, buffer_size);
+	if (len >= buffer_size) {
+		pr_warn("init_uevent_argv: buffer size of %d too small, needed %d\n",
+			buffer_size, len);
 		return -ENOMEM;
 	}