f2fs: add f2fs_ioc_get_compress_blocks

This patch adds f2fs_ioc_get_compress_blocks() to provide a common
f2fs_get_compress_blocks().

Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>