write_lock migrate_disable pushdown to rt_write_lock

pushdown of migrate_disable/enable from write_*lock* to the rt_write_*lock*
api level

general mapping of write_*lock* to mutexes:

write_*lock*
  `-> rt_write_*lock*
          `-> __spin_lock (the sleeping __spin_lock)
                 `-> rt_mutex

write_*lock*s are non-recursive so we have two lock chains to consider
 - write_trylock*/write_unlock
 - write_lock*/wirte_unlock
for both paths the migration_disable/enable must be balanced.

write_trylock* mapping:

write_trylock_irqsave
                `-> rt_write_trylock_irqsave
write_trylock             \
          `-------->  rt_write_trylock
                       ret = rt_mutex_trylock
                              rt_mutex_fasttrylock
                               rt_mutex_cmpxchg
                       if (ret)
                            migrate_disable

write_lock* mapping:

                  write_lock_irqsave
                                `-> rt_write_lock_irqsave
write_lock_irq -> write_lock ----.     \
                  write_lock_bh -+      \
                                 `-> rt_write_lock
                                      __rt_spin_lock()
                                       rt_spin_lock_fastlock()
                                        rt_mutex_cmpxchg()
                                     migrate_disable()

write_unlock* mapping:

                    write_unlock_irqrestore.
                    write_unlock_bh -------+
write_unlock_irq -> write_unlock ----------+
                                           `-> rt_write_unlock()
                                                __rt_spin_unlock()
                                                 rt_spin_lock_fastunlock()
                                                  rt_mutex_cmpxchg()
                                               migrate_enable()

So calls to migrate_disable/enable() are better placed at the rt_write_*
level of lock/trylock/unlock as all of the write_*lock* API has this as a
common path.

This approach to write_*_bh also eliminates the concerns raised with
regards to api inbalances (write_lock_bh -> write_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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