ext4: fix clang build regression

Arnd Bergmann <arnd@arndb.de>

As Stefan pointed out, I misremembered what clang can do specifically,
and it turns out that the variable-length array at the end of the
structure did not work (a flexible array would have worked here
but not solved the problem):

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];

This reverts part of my previous patch, using a fixed-size array
again, but keeping the check for the array overflow.

Fixes: 2df2c340 ("ext4: fix warning about stack corruption")
Reported-by: Stefan Agner <stefan@agner.ch>
Tested-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>