When copying in a struct msghdr from the user, if the user has set the
msg_namelen parameter to a negative value it gets clamped to a valid
size due to a comparison between signed and unsigned values.

Ensure the syscall errors when the user passes in a negative value.
Signed-off-by: NMatthew Leach <matthew.leach@arm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>