Add function to track free bytes in the coremap.

kern/include/vm.h

@@ -55,6 +55,14 @@ int vm_fault(int faulttype, vaddr_t faultaddress);
 vaddr_t alloc_kpages(unsigned npages);
 void free_kpages(vaddr_t addr);
 
+/*
+ * Return amount of free memory (in bytes) available in unallocated coremap pages.
+ * If there are ongoing allocations, this value could change after it is
+ * returned to the caller. But it should have been correct at some point in
+ * time.
+ */
+unsigned int coremap_free_bytes(void);
+
 /* TLB shootdown handling called from interprocessor_interrupt */
 void vm_tlbshootdown_all(void);
 void vm_tlbshootdown(const struct tlbshootdown *);