IB/mlx4: Fix data corruption triggered by wrong headroom marking order

This is an addendum to commit 0e6e7416 ("IB/mlx4: Handle new FW
requirement for send request prefetching").  We also need to handle
prefetch marking properly for S/G segments, or else the HCA may end up
processing S/G segments that are not fully written and end up sending
the wrong data.  This can actually cause data corruption in practice,
especially on systems with relatively slow CPUs (where the HCA is more
likely to prefetch while the CPU is in the middle of writing a work
request into memory).

We write S/G segments in reverse order into the WQE, in order to
guarantee that the first dword of all cachelines containing S/G
segments is written last (overwriting the headroom invalidation
pattern).  The entire cacheline will thus contain valid data when the
invalidation pattern is overwritten.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>