Bug was introduced by cset 1.1659.14.1. Before it server was silently
ignoring that lock can't be acquired because it already acquired.

This patch makes make_global_read_lock_block_commit() return without error
if lock already acquired.


mysql-test/t/flush_table.test:
  Test case for bug#11934 FLUSH TABLES WITH READ LOCK hangs client.
mysql-test/r/flush_table.result:
  Test case for bug#11934 FLUSH TABLES WITH READ LOCK hangs client.
sql/lock.cc:
  Fix bug #11934 Two sequential FLUSH TABLES WITH READ LOCK hangs client.
  Make make_global_read_lock_block_commit() return without error if lock already acquired.