fs/dcache: Move wakeup out of i_seq_dir write held region.

__d_add() and __d_move() wake up waiters on dentry::d_wait from within
the i_seq_dir write held region.  This violates the PREEMPT_RT
constraints as the wake up acquires wait_queue_head::lock which is a
"sleeping" spinlock on RT.

There is no requirement to do so. __d_lookup_unhash() has cleared
DCACHE_PAR_LOOKUP and dentry::d_wait and returned the now unreachable wait
queue head pointer to the caller, so the actual wake up can be postponed
until the i_dir_seq write side critical section is left. The only
requirement is that dentry::lock is held across the whole sequence
including the wake up. The previous commit includes an analysis why this
is considered safe.

Move the wake up past end_dir_add() which leaves the i_dir_seq write side
critical section and enables preemption.

For non RT kernels there is no difference because preemption is still
disabled due to dentry::lock being held, but it shortens the time between
wake up and unlocking dentry::lock, which reduces the contention for the
woken up waiter.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>