rbd: remove VLA usage

As part of the effort to remove VLAs from the kernel[1], this moves
the literal values into the stack array calculation instead of using a
variable for the sizing. The resulting size can be found from
sizeof(buf).

[1] https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Kyle Spiers <kyle@spiers.me>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>