socket: use ki_nbytes instead of iov_length()

This field already contains the length of the iovec, no need to calculate it
again.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>