vm: move vm_cnt to __read_mostly now that it is not written to

While here whack unused locking keys for the struct.

Discussed with:		jeff

sys/sys/vmmeter.h

@@ -73,11 +73,8 @@ struct vmtotal {
 /*
  * System wide statistics counters.
  * Locking:
- *      a - locked by atomic operations
  *      c - constant after initialization
- *      f - locked by vm_page_queue_free_mtx
  *      p - uses counter(9)
- *      q - changes are synchronized by the corresponding vm_pagequeue lock
  */
 struct vmmeter {
 	/*

sys/vm/vm_meter.c

@@ -58,7 +58,7 @@ __FBSDID("$FreeBSD$");
 #include <vm/vm_object.h>
 #include <sys/sysctl.h>
 
-struct vmmeter __exclusive_cache_line vm_cnt = {
+struct vmmeter __read_mostly vm_cnt = {
 	.v_swtch = EARLY_COUNTER,
 	.v_trap = EARLY_COUNTER,
 	.v_syscall = EARLY_COUNTER,