rocksdb: pull up to master

This includes a few patches from Willem Jan Withagen
<wjw@digiware.nl> that fix the build on FreeBSD.

Signed-off-by: Sage Weil <sage@redhat.com>

diff --git a/src/rocksdb b/src/rocksdb
index 936973d1452b773cd110e2d8605572d7a883e644..5647fa427c4eab87c0caf8bcbb91f5846a4623f9 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit 936973d1452b773cd110e2d8605572d7a883e644
+Subproject commit 5647fa427c4eab87c0caf8bcbb91f5846a4623f9