erofs-utils: lib: treat data blocks filled with 0s as a hole

erofs-utils: lib: treat data blocks filled with 0s as a hole

Add optimization to treat data blocks filled with 0s as a hole.
Even though diskspace savings are comparable to chunk based or dedupe,
having no block assigned saves us redundant disk IOs during read.

To detect blocks filled with zeros during chunking, we insert block
filled with zeros (zerochunk) in the hashmap. If we detect a possible
dedupe, we map it to the hole so there is no physical block assigned.

Signed-off-by: Sandeep Dhavale <dhavale@google.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240417234845.2758882-1-dhavale@google.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>