Use panic macro where suitable

kern/kalloc.c

@@ -44,7 +44,7 @@ void kfree(void *pa) {
     // Assert that page is a ligned to a page boundary and that its correctly
     // sized
     if (((u64)pa % PGSIZE) != 0 || (char *)pa < kernel_end || (u64)pa >= PHYSTOP)
-        panic("kfree");
+        PANIC("kfree");
 
     // Fill with junk to catch dangling refs.
     memset(pa, 1, PGSIZE);

kern/libkern/spinlock.c

@@ -64,10 +64,10 @@ uint32_t pop_off(void) {
     Cpu *cpu = mycpu();
 
     if (intr_get())
-        panic("pop_off - interruptible");
+        PANIC("pop_off - interruptible");
 
     if (cpu->noff < 1)
-        panic("pop_off");
+        PANIC("pop_off");
 
     cpu->noff -= 1;