rocksdb: Updated to v6.8.1

We require
https://github.com/facebook/rocksdb/commit/debc4ef18b2e61581ca81dc7fc997b085eb015d9
to build rocksdb_env_librados_test since the bufferlist::copy() method
was removed in c724369

Signed-off-by: Brad Hubbard <bhubbard@redhat.com>

diff --git a/src/rocksdb b/src/rocksdb
index 4c736f177851cbf9fb7a6790282306ffac5065f8..f86d378860f399bf595bd1135f39f03a9eaca59f 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit 4c736f177851cbf9fb7a6790282306ffac5065f8
+Subproject commit f86d378860f399bf595bd1135f39f03a9eaca59f