[IB] mthca: Round up number of slots in HCA context memory table

When allocating a table for mem-free HCA context, don't assume that
obj_size * nobj is an even multiple of MTHCA_TABLE_CHUNK_SIZE.  In
particular, make sure we allocate at least one slot even if the table
is smaller than MTHCA_TABLE_CHUNK_SIZE.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>