net: use SYSCALL_DEFINEx for sys_recv

Make sys_recv a first class citizen by using the SYSCALL_DEFINEx
macro. Besides being cleaner this will also generate meta data
for the system call so tracing tools like ftrace or LTTng can
resolve this system call.

Signed-off-by: Jan Glauber <jan.glauber@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>