erofs: decouple `struct erofs_anon_fs_type`

erofs: decouple `struct erofs_anon_fs_type`

  - Move the `struct erofs_anon_fs_type` to super.c and expose it
    in preparation for the upcoming page cache share feature;

  - Remove the `.owner` field, as they are all internal mounts and
    fully managed by EROFS.  Retaining `.owner` would unnecessarily
    increment module reference counts, preventing the EROFS kernel
    module from being unloaded.

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