IN OS_THREAD_EQ

rb://977
approved by: Marko Makela

rw_lock::writer_thread field contains the thread id of current x-holder
or wait-x thread. This field is un-initialized at lock creation and is
written to for the first time when an attempt is made to x-lock.

Current code considers ::writer_thread as valid memory region only when
the lock is held in x-mode (or there is an x-waiter). This is an
overkill and it generates valgrind warnings.

The fix is to consider ::writer_thread as valid memory region once it
has been written to.

Reasoning:
==========
The ::writer_thread can be safely considered valid because:

* We only ever do comparison with current calling threads id.
* We only ever do comparison when ::recursive flag is set
* We always unset ::recursive flag in x-unlock
* Same thread cannot be unlocking and attempting to lock at the same
time
* thread_id recycling is not an issue because before an id is recycled
the thread must leave innodb meaning it must release all locks meaning
it must unset ::recursive flag.