svcrpc: store gss mech in svc_cred

Store a pointer to the gss mechanism used in the rq_cred and cl_cred.
This will make it easier to enforce SP4_MACH_CRED, which needs to
compare the mechanism used on the exchange_id with that used on
protected operations.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 72f0c4e..109b4340 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -1188,6 +1188,9 @@
 	target->cr_gid = source->cr_gid;
 	target->cr_group_info = source->cr_group_info;
 	get_group_info(target->cr_group_info);
+	target->cr_gss_mech = source->cr_gss_mech;
+	if (source->cr_gss_mech)
+		gss_mech_get(source->cr_gss_mech);
 	return 0;
 }