Simplify away recursive flag: it is not necessary for rw-locks to operate
properly. Now writer_thread != 0 means recursive.

As we only need correct value of writer_thread only in writer_thread itself
it is rather safe to load and update it non-atomically.

This patch also fixes potential reorder of "writer_thread" and "recursive"
loads (aka MDEV-7148), which was reopened along with InnoDB thread fences
simplification. Previous versions are unaffected, because they have os_rmb
in rw_lock_lock_word_decr(). It wasn't observed at the moment of writing
though.