d_drop() must use the dentry->d_lock spinlock.  In some cases __d_drop()
was used without holding the dentry->d_lock spinlock, too.  This could end
in a race with __d_lookup().
Signed-off-by: Jan Blunck <j.blunck@tu-harburg.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>