RFC: Free cred resources associated with Manage_gids

NFS4 may call get_req_uid_gid multiple times. Free resources
associated with old cred before updating the cred. If the
supplementary groups are created with Manage_Gids, they are
unreferenced in get_req_uid_gid prior to updating.

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>
Avoid double free!

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>