Define f2fs_listxattr and to NULL when CONFIG_F2FS_FS_XATTR is not
enabled, then we can remove many ugly ifdef macros in the code.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>