syscall: Drop #if 0'd sendfile code

Signed-off-by: Asias He <asias@cloudius-systems.com>
net: Get rid of unnecessary recursive locking

We already get SOCK_LOCK(so) before we call sblock() everywhere. No need
to do recursive locking.

Similarly, we will call SOCK_UNLOCK(so) after we call sbunlock()
everywhere.

Signed-off-by: Asias He <asias@cloudius-systems.com>