Check integer overflow in do_insnlist_ioctl()

Check integer overflow in do_insnlist_ioctl()

Follow patch to "staging" comedi kernel sources by Dan Carpenter.

There is an integer overflow here that could cause memory corruption
on 32 bit systems.

insnlist.n_insns could be a very high value size calculation for
kmalloc() could overflow resulting in a smaller "insns" than
expected.  In the for (i = 0; i < insnlist.n_insns; i++) {... loop
we would read past the end of the buffer, possibly corrupting memory
as well.