erofs: code cleanup by removing ifdef macro surrounding

Define erofs_listxattr and erofs_xattr_handlers to NULL when
CONFIG_EROFS_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: Gao Xiang <hsiangkao@redhat.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200526090343.22794-1-cgxu519@mykernel.net


Signed-off-by: Gao Xiang <hsiangkao@redhat.com>