comedi: Fail COMEDI_INSNLIST ioctl if n_insns is too large

In the Linux kernel, the following vulnerability has been resolved:

comedi: Fail COMEDI_INSNLIST ioctl if n_insns is too large

The handling of the COMEDI_INSNLIST ioctl allocates a kernel buffer to hold the array of struct comedi_insn, getting the length from the n_insns member of the struct comedi_insnlist supplied by the user. The allocation will fail with a WARNING and a stack dump if it is too large.

Avoid that by failing with an -EINVAL error if the supplied n_insns value is unreasonable.

Define the limit on the n_insns value in the MAX_INSNS macro. Set this to the same value as MAX_SAMPLES (65536), which is the maximum allowed sum of the values of the member n in the array of struct comedi_insn, and sensible comedi instructions will have an n of at least 1.