netlabel: Fix compiler warnings in netlabel_mgmt.c

Fix the compiler warnings below, thanks to Andrew Morton for finding them.

 net/netlabel/netlabel_mgmt.c: In function `netlbl_mgmt_listentry':
 net/netlabel/netlabel_mgmt.c:268: warning: 'ret_val' might be used
  uninitialized in this function

Signed-off-by: Paul Moore <paul.moore@hp.com>