staging: erofs: rename data_mapping_mode to datamode

data_mapping_mode is too long as a member name of erofs_vnode,
datamode is straight-forward enough.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>