erofs: add on-disk definition for metadata compression

Filesystem metadata has a high degree of redundancy, so it should
compress well in the general case.

Although metadata compression can increase overall I/O latency, many
users care more about minimized image sizes than extreme runtime
performance.  Let's implement metadata compression in response to user
requests [1].

Actually, it's quite simple to implement metadata compression: since
EROFS already supports per-inode compression, we can simply treat a
special inode (called `the metabox inode`) as a container for compressed
inode metadata.  Since EROFS supports multiple algorithms, users can
even specify LZ4 for metadata and LZMA for data.

To better support incremental builds, the MSB of NIDs indicates where
the inode metadata is located: if bit 63 is set, the inode itself should
be read from `the metabox inode`.

Optionally, shared xattrs can also be kept in `the metabox inode` if
COMPAT_SHARED_EA_IN_METABOX is set.

[1] https://issues.redhat.com/browse/RHEL-75783



Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Acked-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20250717070804.1446345-2-hsiangkao@linux.alibaba.com