use ffi.cast for type convention and add sample script skb_cast.kp

Example:
	trace net:netif_receive_skb {
        	var skb = ffi.cast("struct sk_buff *", arg1)
		print(skb.len, skb.data_len, skb.skb_iif)
	}

Now ktap can support type convention functionality, this is
a big step for ktap.

In future, there will don't need to declare 'struct sk_buff'
anymore in ktap script.

This patch also kill ffi.free, all memory allocated by ffi.new
will be free automatically when ktap thread exit.

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>
add 'reload' build target in Makefile

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>