[NET]: Fix net/socket.c warnings.

GCC (correctly) says:

net/socket.c: In function ‘sys_sendto’:
net/socket.c:1510: warning: ‘err’ may be used uninitialized in this function
net/socket.c: In function ‘sys_recvfrom’:
net/socket.c:1571: warning: ‘err’ may be used uninitialized in this function

sock_from_file() either returns filp->private_data or it
sets *err and returns NULL.

Callers return "err" on NULL, but filp->private_data could
be NULL.

Some minor rearrangements of error handling in sys_sendto
and sys_recvfrom solves the issue.

Signed-off-by: David S. Miller <davem@davemloft.net>