erofs-utils: mkfs: do not enable lz4_0padding for plain images

This is a new regression introduced in erofs-utils 1.9.

Fixes: 16a77fed8511 ("erofs-utils: lib: unexport z_erofs_compress_{init,exit}")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20260225141821.668836-1-hsiangkao@linux.alibaba.com