net: x25: Use list_for_each_entry() to simplify code in x25_link.c

Convert list_for_each() to list_for_each_entry() where
applicable. This simplifies the code.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>