rocksdb: sync with upstream

since
https://github.com/facebook/rocksdb/pull/3862 has been merged by
upstream.

Fixes: http://tracker.ceph.com/issues/23653
Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/rocksdb b/src/rocksdb
index f4a857da0b720691effc524469f6db895ad00d8e..bc7e8d472e7d37a33f3f45dec393769011ec1e08 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit f4a857da0b720691effc524469f6db895ad00d8e
+Subproject commit bc7e8d472e7d37a33f3f45dec393769011ec1e08