net: socket: replace call to sys_recv() with __sys_recvfrom()

sys_recv() merely expands the parameters to __sys_recvfrom() by NULL and
NULL. Open-code this in the two places which used sys_recv() as a wrapper
to __sys_recvfrom().

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.GA17492@light.dominikbrodowski.net



Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>