f2fs: rebuild compressed extent cache update code for readonly-fs · Pull Request !152 · openEuler/kernel

For readonly format image, we first rebuild the code of updating extent, and then support caching extent for no-compressed file.
Its idea is almost same with original design. At last we remove the limit for no-compressed file that enable it use extent.
The motivation of rebuilding comes from:
 a) If we don't enable the compress config, it will build some unnecessary code into target in f2fs_get_dnode_of_data
b) The f2fs_cluster_blocks_are_contiguous is called only when updating compressed extent cache and being called once in extent_cache.c
So, we rebuild it by adding f2fs_readonly_update_extent_cache() to update compressed extent cache in readonly-fs and moving
f2fs_update_extent_tree_range_compressed and f2fs_cluster_blocks_are_contiguous into extent_cache.c. Both function can be implemented internally in extent_cache.c.
No-compressed file may suffer read performance issue due to it can't use extent cache or the largest extent in inode can't covered other parts of continuous blocks in readonly format f2fs image.
Now it won't build extent cacge tree for no-compressed file in readonly format f2fs image.

For readonly format f2fs image, maybe the no-compressed file don't have the largest extent, or it have more than one part which have internally continuous blocks. So we add extent cache tree for the no-compressed file in readonly format f2fs image.

The cache policy is almost same with compressed file. The difference is that, the no-compressed file part will set min-number of continuous blocks 2FS_MIN_EXTENT_LEN in order to reduce cache  fragmentation.

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