Before this patch the build with libstdc++ hangs for
00967_insert_into_distributed_different_types test (and I guess some
others), this is because the rwlock is acquired from different thread as
it was unlocked which causes UB [1], fix this by moving unlock into
writeSuffix().

  [1]: The pthread_rwlock_unlock() function shall release a lock held on
       the read-write lock object referenced by rwlock. Results are undefined
       if the read-write lock rwlock is not held by the **calling thread**.