nfsd4: backchannel should use client-provided security flavor

For now this only adds support for AUTH_NULL.  (Previously we assumed
AUTH_UNIX.)  We'll also need AUTH_GSS, which is trickier.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 0fd342a..0498053 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -151,6 +151,7 @@
 };
 
 struct nfsd4_cb_sec {
+	u32	flavor; /* (u32)(-1) used to mean "no valid flavor" */
 	u32	uid;
 	u32	gid;
 };