Fix unsigned integer underflow in fd-trans.c

Fix unsigned integer underflow in fd-trans.c

In any of these `*_for_each_*` functions, the last entry in the buffer (so the
"remaining length in the buffer" `len` is equal to the length of the
entry `nlmsg_len`/`nla_len`/etc) has size that is not a multiple of the
alignment, the aligned lengths `*_ALIGN(*_len)` will be greater than `len`.
Since `len` is unsigned (`size_t`), it underflows and the loop will read
pass the buffer.

This may manifest as random EINVAL or EOPNOTSUPP error on IO or network
system calls.

Signed-off-by: Shu-Chun Weng <scw@google.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20191018001920.178283-1-scw@google.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>