blk-mq: use struct_size() in kmalloc()

blk-mq: use struct_size() in kmalloc()

Change struct size calculation to use struct_size()
to align with new recommended practices[1] which quotes:
"Another common case to avoid is calculating the size of a structure with
a trailing array of others structures, as in:

header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
                 GFP_KERNEL);

Instead, use the helper:

header = kzalloc(struct_size(header, item, count), GFP_KERNEL);"

Signed-off-by: Mehdi Ben Hadj Khelifa <mehdi.benhadjkhelifa@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>