read_lock migrate_disable pushdown to rt_read_lock

pushdown of migrate_disable/enable from read_*lock* to the rt_read_*lock*
api level

general mapping to mutexes:

read_*lock*
  `-> rt_read_*lock*
          `-> __spin_lock (the sleeping spin locks)
                 `-> rt_mutex

The real read_lock* mapping:

          read_lock_irqsave -.
read_lock_irq                `-> rt_read_lock_irqsave()
       `->read_lock ---------.       \
          read_lock_bh ------+        \
                             `--> rt_read_lock()
                                   if (rt_mutex_owner(lock) != current){
                                           `-> __rt_spin_lock()
                                                rt_spin_lock_fastlock()
                                                       `->rt_mutex_cmpxchg()
                                    migrate_disable()
                                   }
                                   rwlock->read_depth++;
read_trylock mapping:

read_trylock
          `-> rt_read_trylock
               if (rt_mutex_owner(lock) != current){
                                              `-> rt_mutex_trylock()
                                                   rt_mutex_fasttrylock()
                                                    rt_mutex_cmpxchg()
                migrate_disable()
               }
               rwlock->read_depth++;

read_unlock* mapping:

read_unlock_bh --------+
read_unlock_irq -------+
read_unlock_irqrestore +
read_unlock -----------+
                       `-> rt_read_unlock()
                            if(--rwlock->read_depth==0){
                                      `-> __rt_spin_unlock()
                                           rt_spin_lock_fastunlock()
                                                        `-> rt_mutex_cmpxchg()
                             migrate_disable()
                            }

So calls to migrate_disable/enable() are better placed at the rt_read_*
level of lock/trylock/unlock as all of the read_*lock* API has this as a
common path. In the rt_read* API of lock/trylock/unlock the nesting level
is already being recorded in rwlock->read_depth, so we can push down the
migrate disable/enable to that level and condition it on the read_depth
going from 0 to 1 -> migrate_disable and 1 to 0 -> migrate_enable. This
eliminates the recursive calls that were needed when migrate_disable/enable
was done at the read_*lock* level.

The approach to read_*_bh also eliminates the concerns raised with the
regards to api inbalances (read_lock_bh -> read_unlock+local_bh_enable)

Tested-by: Carsten Emde <C.Emde@osadl.org>
Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

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