watchdog: fix double lock in watchdog_nmi_enable_all

Commit ab992dc3 ("watchdog: Fix merge 'conflict'") has introduced an
obvious deadlock because of a typo.  watchdog_proc_mutex should be
unlocked on exit.

Thanks to Miroslav Benes who was staring at the code with me and noticed
this.
Signed-off-by: NMichal Hocko <mhocko@suse.cz>
Duh-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/watchdog.c

@@ -621,7 +621,7 @@ void watchdog_nmi_enable_all(void)
 	put_online_cpus();
 
 unlock:
-	mutex_lock(&watchdog_proc_mutex);
+	mutex_unlock(&watchdog_proc_mutex);
 }
 
 void watchdog_nmi_disable_all(void)