cifs: don't pop a printk when sending on a socket is interrupted

If we kill the process while it's sending on a socket then the
kernel_sendmsg will return -EINTR. This is normal. No need to spam the
ring buffer with this info.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>