totempg: Fix corrupted messages

totempg: Fix corrupted messages

Commit 899cb299831fea479ca8bc64d99fb1fce215d795 changed copy_len
to iovec[i].iov_len, assuming,
copy_len is always the same as iovec[i].iov_len under those
circumstances, but it missed the possability of small message being
partly put at the end of packet, which cuts this message in two parts
and therefore making copy_len not equal to iovec[i].iov_len.

This is revert of 899cb299831fea479ca8bc64d99fb1fce215d795

Signed-off-by: Rytis Karpuška <rytisk@neurotechnology.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>