staging: erofs: fix dummy functions erofs_{get, list}xattr

dummy functions erofs_{get,list}xattr should be inlined
without xattr enabled.

Signed-off-by: Yue Hu <zbestahu@gmail.com>
[ Gao Xiang : this patch was "staging: erofs: remove needless
              dummy functions of erofs_{get,list}xattr. "]
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-4-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>