rocksdb: revert the change introduced by dc41731

dc41731 reverts the submodule reference of rocksdb, and point it to an
older version which fails to support ppc64le and AArch64.

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/rocksdb b/src/rocksdb
index 5647fa427c4eab87c0caf8bcbb91f5846a4623f9..6797e6ffacd90470140f64cea068970bb3a30aaf 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit 5647fa427c4eab87c0caf8bcbb91f5846a4623f9
+Subproject commit 6797e6ffacd90470140f64cea068970bb3a30aaf