More XtraDB after-merge fixes following review and buildbot runs:

- Better fix for --innodb-use-sys-malloc causing Valgrind warnings.
 - Different fix for INNODB_IBUF_MAX_SIZE variable changing default value.
 - Fix some problems with the safe mutex lazy init patch.

mysql-test/include/mtr_check.sql:
  Do not check INNODB_IBUF_MAX_SIZE for changes. It is not a dynamic variable, so cannot
  be changed by a test case anyway, and the value may vary slightly from one start of the
  server to the next.
mysql-test/lib/mtr_cases.pm:
  Even just starting and stopping the server with --innodb-use-sys-malloc to check for
  disabled test case under valgrind will cause valgrind leak warnings. So add not_valgrind
  to the list of conditions also tested for directly in mysql-test-run.pl.
mysql-test/mysql-test-run.pl:
  Even just starting and stopping the server with --innodb-use-sys-malloc to check for
  disabled test case under valgrind will cause valgrind leak warnings. So add not_valgrind
  to the list of conditions also tested for directly in mysql-test-run.pl.
mysys/thr_mutex.c:
  Fix a few problems found during review of the lazy init safe mutex patch.
storage/xtradb/ibuf/ibuf0ibuf.c:
  Revert previous fix of INNODB_IBUF_MAX_SIZE default varying slightly between server starts.
  (Fixed instead by ignoring that variable in the test suite).

mysql-test/mysql-test-run.pl

@@ -224,7 +224,7 @@ my $opt_strace_client;
 our $opt_user = "root";
 
 my $opt_valgrind= 0;
-my $opt_valgrind_mysqld= 0;
+our $opt_valgrind_mysqld= 0;
 my $opt_valgrind_mysqltest= 0;
 my @default_valgrind_args= ("--show-reachable=yes");
 my @valgrind_args;