net: socket: add __sys_sendto() helper; remove in-kernel call to syscall

net: socket: add __sys_sendto() helper; remove in-kernel call to syscall

Using the net-internal helper __sys_sendto() allows us to avoid the
internal calls to the sys_sendto() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527[email protected]

Cc: David S. Miller <[email protected]>
Cc: [email protected]
Signed-off-by: Dominik Brodowski <[email protected]>