ext4: fix stack memory corruption with 64k block size

ext4: fix stack memory corruption with 64k block size

The number of 'counters' elements needed in 'struct sg' is
super_block->s_blocksize_bits + 2. Presently we have 16 'counters'
elements in the array. This is insufficient for block sizes >= 32k. In
such cases the memcpy operation performed in ext4_mb_seq_groups_show()
would cause stack memory corruption.

Fixes: c9de560ded61f
Signed-off-by: Chandan Rajendra <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: [email protected]