Remove the acquisition and release of Giant around pmap_growkernel().

It's unnecessary for two reasons: (1) Giant is at present already held in
such cases and (2) our various implementations of pmap_growkernel() look to
be MP safe.  (For example, for sparc64 the proof of (2) is trivial.)

sys/vm/vm_map.c

@@ -965,9 +965,7 @@ vm_map_findspace(
 	if (map == kernel_map) {
 		vm_offset_t ksize;
 		if ((ksize = round_page(start + length)) > kernel_vm_end) {
-			mtx_lock(&Giant);
 			pmap_growkernel(ksize);
-			mtx_unlock(&Giant);
 		}
 	}
 	return (0);