btl/openib: allow the use of the openib btl in thread muliple

There were several issues preventing the openib btl from running in
thread multiple mode:

 - Missing locks in UDCM when generating a loopback endpoint. Fixed in
   open-mpi/ompi@8205d79819.

 - Incorrect sequence numbers generated in debug mode. This did not
   prevent the openib btl from running but instead produced incorrect
   error messages in debug builds.

 - Recursive locking of the rcache lock caused by the malloc
   hooks. This is fixed by open-mpi/ompi#827

Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>

opal/mca/btl/openib/btl_openib_component.c

@@ -2534,12 +2534,6 @@ btl_openib_component_init(int *num_btl_modules,
         malloc_hook_set = true;
     }
 #endif
-    /* Currently refuse to run if MPI_THREAD_MULTIPLE is enabled */
-    if (enable_mpi_threads && !mca_btl_base_thread_multiple_override) {
-        opal_output_verbose(5, opal_btl_base_framework.framework_output,
-                            "btl:openib: MPI_THREAD_MULTIPLE not suppported; skipping this component");
-        goto no_btls;
-    }
 
     /* Per https://svn.open-mpi.org/trac/ompi/ticket/1305, check to
        see if $sysfsdir/class/infiniband exists.  If it does not,