erofs-utils: lib: use bitmaps to accelerate bucket selection

erofs-utils: lib: use bitmaps to accelerate bucket selection

Instead of looping over bucket lists directly, maintain bitmaps
for more efficient greedy selection.

ILSVRC2012_img_train.tar (1,281,168 inodes) [1]
  uncompressed EROFS (vanilla):       147,059,949,568B  36m29.529s
  uncompressed EROFS (patched):       147,059,511,296B  1m14.920s

ILSVRC2012_img_val.tar (50,001 inodes)  6,744,924,160B [2]
  uncompressed EROFS (vanilla):         6,713,278,464B  29.998s
  uncompressed EROFS (patched):         6,713,188,352B  23.753s

[1] https://image-net.org/data/ILSVRC/2012/ILSVRC2012_img_train.tar
    $ mkdir train; tar -xOf ILSVRC2012_img_train.tar | tar -xi -C train

[2] https://image-net.org/data/ILSVRC/2012/ILSVRC2012_img_vol.tar
    $ mkfs.erofs --tar=f --sort none foo.erofs ILSVRC2012_img_vol.tar

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250103090338.740593-5-hsiangkao@linux.alibaba.com