erofs: rename lz4_0pading to zero_padding

Renaming lz4_0padding to zero_padding globally since LZMA and later
algorithms also need that.

Link: https://lore.kernel.org/r/20211112160935.19394-1-jnhuang95@gmail.com


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