Kernel: {Mutex,Spinlock}::own_lock() => is_locked_by_current_thread()

Rename these API's to make it more clear what they are checking.

Kernel/Syscall.cpp

@@ -236,7 +236,7 @@ NEVER_INLINE void syscall_handler(TrapFrame* trap)
     // Check if we're supposed to return to userspace or just die.
     current_thread->die_if_needed();
 
-    VERIFY(!g_scheduler_lock.own_lock());
+    VERIFY(!g_scheduler_lock.is_locked_by_current_thread());
 }
 
 }