Drop Giant before sleeping on a busy page.

Before the page busy code was converted to make direct use of
sleepqueues, this was handled by _sleep().

Reported by:	glebius
Reviewed by:	kib
Sponsored by:	The FreeBSD Foundation

sys/vm/vm_page.c

@@ -1053,8 +1053,10 @@ _vm_page_busy_sleep(vm_object_t obj, vm_page_t m, const char *wmesg,
 	}
 	if (locked)
 		VM_OBJECT_DROP(obj);
+	DROP_GIANT();
 	sleepq_add(m, NULL, wmesg, 0, 0);
 	sleepq_wait(m, PVM);
+	PICKUP_GIANT();
 }
 
 /*