IB/mthca: Add missing sg_init_table() in mthca_map_user_db()

Usually harmless, since the scatterlist is always hard-coded to a length
of 1, but it triggers a BUG() if CONFIG_DEBUG_SG=y, so we better fix it.
This fixes <http://bugzilla.kernel.org/show_bug.cgi?id=9934>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>