erofs: leave compressed inodes unsupported in fscache mode for now

erofs over fscache doesn't support the compressed layout yet. It will
cause NULL crash if there are compressed inodes contained when working
in fscache mode.

So far in the erofs based container image distribution scenarios
(RAFS v6), the compressed RAFS v6 images are downloaded and then
decompressed on demand as an uncompressed erofs image. Then the erofs
image is mounted in fscache mode for containers to use. IOWs, currently
compressed data is decompressed on the userspace side instead and
uncompressed erofs images will be finally cached.

The fscache support for the compressed layout is still under
development and it will be used for runtime decompression feature.
Anyway, to avoid the potential crash, let's leave the compressed inodes
unsupported in fscache mode until we support it later.

Fixes: 1442b02 ("erofs: implement fscache-based data read for non-inline layout")
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220526010344.118493-1-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/inode.c

@@ -288,7 +288,10 @@ static int erofs_fill_inode(struct inode *inode, int isdir)
 	}
 
 	if (erofs_inode_is_data_compressed(vi->datalayout)) {
-		err = z_erofs_fill_inode(inode);
+		if (!erofs_is_fscache_mode(inode->i_sb))
+			err = z_erofs_fill_inode(inode);
+		else
+			err = -EOPNOTSUPP;
 		goto out_unlock;
 	}
 	inode->i_mapping->a_ops = &erofs_raw_access_aops;