IB/mad: Check hop count field in directed route MAD to avoid array overflow

The hop count field in a directed route MAD is only allowed to be in the
range 0 to 63 (by spec).  Check that this really is the case to avoid
accessing outside the bounds of the hop array.

Reported-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>