ext4: avoid variable size array in structs

ext4: avoid variable size array in structs

Clang does not support variable length array in structure, even
if the array is at the very end. Avoid variable sized array by
using the maximum length again. This fixes clang build error:

fs/ext4/mballoc.c:2303:17: error: fields must have a constant size: 'variable length array in structure' extension will never be supported
                ext4_grpblk_t counters[blocksize_bits + 2];

Fixes: 2df2c3402fc8 ("ext4: fix warning about stack corruption")
Signed-off-by: Stefan Agner <stefan@agner.ch>