net: make __sys_accept4_file() static

__sys_accept4_file() isn't used outside of the file, make it static.

As the same time, move file_flags and nofile parameters into
__sys_accept4_file().

Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Signed-off-by: David S. Miller <davem@davemloft.net>