- Only lock/unlock when using threads.

Basically revert this part of r16015.

This commit was SVN r16029.

The following SVN revision numbers were found above:
  r16015 --> open-mpi/ompi@435e7d80e9

ompi/mca/btl/sm/btl_sm_fifo.h

@@ -10,12 +10,14 @@ do {                                                                        \
     fifo=&(mca_btl_sm_component.fifo[peer_smp_rank][my_smp_rank]);          \
                                                                             \
     /* thread lock */                                                       \
+    if(opal_using_threads())                                                \
         opal_atomic_lock(fifo->head_lock);                                  \
     /* post fragment */                                                     \
     while(ompi_fifo_write_to_head(hdr, fifo,                                \
         mca_btl_sm_component.sm_mpool) != OMPI_SUCCESS)                     \
         opal_progress();                                                    \
     MCA_BTL_SM_SIGNAL_PEER(endpoint_peer);                                  \
+    if(opal_using_threads())                                                \
         opal_atomic_unlock(fifo->head_lock);                                \
 } while(0)