libnvme: git submodule update

Update libnvme to pull in authentication functions.

diff --git a/libnvme b/libnvme
index 38d5f958a9c557499060f326d68fca47b8da362d..c6d121af85935899cfef20989b2ff3fbf74da69b 160000 (submodule)
--- a/libnvme
+++ b/libnvme
@@ -1 +1 @@
-Subproject commit 38d5f958a9c557499060f326d68fca47b8da362d
+Subproject commit c6d121af85935899cfef20989b2ff3fbf74da69b