Fix eBPF exit instruction

[xxr0ss]

exit means "return" according to Jump instructions

code	value	src	description	notes
BPF_EXIT	0x9	0x0	return	BPF_JMP only

and Program-local functions

A BPF_EXIT within the program-local function will return to the caller.

[xxr0ss]

oh, I think I need to figure out the calculation of return address before this change