libceph: Avoid holding the zero page on ceph_msgr_slab_init errors

ceph_msgr_slab_init may fail due to a temporary ENOMEM.

Delay a bit the initialization of zero_page in ceph_msgr_init and
reorder its cleanup in _ceph_msgr_exit so it's done in reverse
order of setup.

BUG_ON() will not suffer to be postponed in case it is triggered.
Signed-off-by: Benoît Canet <benoit.canet@nodalink.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/messenger.c

@@ -276,22 +276,22 @@ static void _ceph_msgr_exit(void)
 		ceph_msgr_wq = NULL;
 	}
 
-	ceph_msgr_slab_exit();
-
 	BUG_ON(zero_page == NULL);
 	page_cache_release(zero_page);
 	zero_page = NULL;
+
+	ceph_msgr_slab_exit();
 }
 
 int ceph_msgr_init(void)
 {
+	if (ceph_msgr_slab_init())
+		return -ENOMEM;
+
 	BUG_ON(zero_page != NULL);
 	zero_page = ZERO_PAGE(0);
 	page_cache_get(zero_page);
 
-	if (ceph_msgr_slab_init())
-		return -ENOMEM;
-
 	/*
 	 * The number of active work items is limited by the number of
 	 * connections, so leave @max_active at default.