erofs: tidy up synchronous decompression

erofs: tidy up synchronous decompression

 - Get rid of `sbi->opt.max_sync_decompress_pages` since it's fixed as
   3 all the time;

 - Add Z_EROFS_MAX_SYNC_DECOMPRESS_BYTES in bytes instead of in pages,
   since for non-4K pages, 3-page limitation makes no sense;

 - Move `sync_decompress` to sbi to avoid unexpected remount impact;

 - Fold z_erofs_is_sync_decompress() into its caller;

 - Better description of sysfs entry `sync_decompress`.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>