mlx4: Correct error flows in rereg_mr

This patch addresses feedback from Sagi Grimberg on the rereg_mr
implementation of mlx4.  The following are fixed:

1. Set the correct pd_flags
2. Make sure we change the iova and size MR fields only after
   successful write and allocation of the MTTs.
3. Make the error checking more robust

Fixes: e630664c ("mlx4_core: Add helper functions to support MR re-registration")
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>