Backport of revno: 2617.68.13

Introduce a counter for protection against global read lock on thread level.

The functions for protection against global read lock sometimes need a local
variable to signal when the protection is set, and hence need to be released.
It would be better to control this behaviour via a counter on the THD struct,
telling how many times the protection has been claimed by the current thread.
A side-effect of the fix is that if protection is claimed twice for a thread,
only a simple increment is required for the second claim, instead of a
mutex-protected increment of the global variable protect_against_global_read_lock.

sql/sql_class.h

@@ -1874,6 +1874,7 @@ public:
   ulong	     rand_saved_seed1, rand_saved_seed2;
   pthread_t  real_id;                           /* For debugging */
   my_thread_id  thread_id;
+  uint       global_read_lock_protection;// GRL protection count
   uint	     tmp_table, global_read_lock;
   uint	     server_status,open_options;
   enum enum_thread_type system_thread;