Move vm_ndomains to vm.h where it can be used with a single header include

rather than requiring a half-dozen.  Many non-vm files may want to know
the number of valid domains.

Sponsored by:	Netflix, Dell/EMC Isilon

sys/vm/vm.h

@@ -148,6 +148,8 @@ extern void vm_ksubmap_init(struct kva_md_info *);
 
 extern int old_mlock;
 
+extern int vm_ndomains;
+
 struct ucred;
 int swap_reserve(vm_ooffset_t incr);
 int swap_reserve_by_cred(vm_ooffset_t incr, struct ucred *cred);

sys/vm/vm_phys.h

@@ -52,7 +52,6 @@ struct mem_affinity {
 extern struct mem_affinity *mem_affinity;
 extern int *mem_locality;
 #endif
-extern int vm_ndomains;
 
 struct vm_freelist {
 	struct pglist pl;