IB/core: Documentation fix in the MAD header file

In ib_mad.h, ib_mad_snoop_handler uses send_buf rather than send_wr

Signed-off-by: Hal Rosenstock <hal@mellanox.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>