Unconditionally enable debug.vm_lowmem.

It is useful for testing purposes to be able to drain UMA caches, so
do not limit the sysctl to DIAGNOSTIC kernels.

MFC after:	1 week
Sponsored by:	Netflix

sys/vm/vm_kern.c

@@ -839,7 +839,6 @@ kmem_bootstrap_free(vm_offset_t start, vm_size_t size)
 #endif
 }
 
-#ifdef DIAGNOSTIC
 /*
  * Allow userspace to directly trigger the VM drain routine for testing
  * purposes.
@@ -862,4 +861,3 @@ debug_vm_lowmem(SYSCTL_HANDLER_ARGS)
 
 SYSCTL_PROC(_debug, OID_AUTO, vm_lowmem, CTLTYPE_INT | CTLFLAG_RW, 0, 0,
     debug_vm_lowmem, "I", "set to trigger vm_lowmem event with given flags");
-#endif