net: fix kernel-doc warnings in header files

Add missing structure kernel-doc descriptions to sock.h & skbuff.h
to fix kernel-doc warnings.

(I think that Stephen H. sent a similar patch, but I can't find it.
I just want to kill the warnings, with either patch.)

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>