fs/dcache: disable preemption on i_dir_seq's write side

i_dir_seq is an opencoded seqcounter. Based on the code it looks like we
could have two writers in parallel despite the fact that the d_lock is
held. The problem is that during the write process on RT the preemption
is still enabled and if this process is interrupted by a reader with RT
priority then we lock up.
To avoid that lock up I am disabling the preemption during the update.
The rename of i_dir_seq is here to ensure to catch new write sides in
future.

Cc: stable-rt@vger.kernel.org
Reported-by:  <Oleg.Karfich@wago.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>