userspace code test: print the ip address of a packet
userspace code test: elf file test
kernel code reading: more comment in net_dev
kernel code reading: move net namespace related code to another file
kernel code reading: init the hash table in net
qemu code reading: network initialization
kernel code test: hlist_add_head()

Add the function and test for hlist_add_head().

Then iterate on the hlist to dump the elements in the hlist.