IB: Make sure struct ib_user_mad.data is aligned

Make the untyped data region in ib_user_mad have type u64 so that it
gets aligned properly.  This avoids alignment faults in ib_umad when
casting the data field to an rmpp_mad and accessing the 64-bit tid
field on architectures like ia64.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>