syscall: Drop #if 0'd sendfile code

Killing it simplifies the sblock/sbunlock code auditing.

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>
net: Drop sblock and sbunlock

sblock/sbunlock use socket lock (so->so_mtx). We already take the lock
in all the places where sblock is called. We can remove the unnecessary
recursive locking.

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

With the removal of sblock and sbunlock, sockbuf_iolock is not used any
more. Remove it.

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