tcp/syncache: Simplify syncache_add() interface

tcp/syncache: Simplify syncache_add() interface

Since the passed in 'sop' are always NULL upon returning, we do not need
to pass in "struct socket **"; just pass in the listen socket, i.e.
struct socket *.

Nuke the code follows the syncache_add(), which acts upon non-NULL 'sop'
returns from syncache_add().  These codes are actually never executed.