erofs: Replace zero-length array with flexible-array member

There is a regular need in the kernel to provide a way to declare
having a dynamically sized set of trailing elements in a structure.

Kernel code should always use `flexible array members' [1] for these
cases. The older style of one-element or zero-length arrays should
no longer be used [2].

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.15/process/deprecated.html#zero-length-and-one-element-arrays

Link: https://lore.kernel.org/r/20211206121702.221331-1-hsiangkao@linux.alibaba.com


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>