__dentry_kill(): get consistent rules for victim's refcount

Currently we call it with refcount equal to 1 when called from
dentry_kill(); all other callers have it equal to 0.

Make it always be called with zero refcount; on this step we
just decrement it before the calls in dentry_kill().  That is
safe, since all places that care about the value of refcount
either do that under ->d_lock or hold a reference to dentry
in question.  Either is sufficient to prevent observing a
dentry immediately prior to __dentry_kill() getting called
from dentry_kill().

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -729,6 +729,7 @@ static struct dentry *dentry_kill(struct dentry *dentry)
 			goto slow_positive;
 		}
 	}
+	dentry->d_lockref.count--;
 	__dentry_kill(dentry);
 	return parent;
 
@@ -741,6 +742,7 @@ got_locks:
 	if (unlikely(dentry->d_lockref.count != 1)) {
 		dentry->d_lockref.count--;
 	} else if (likely(!retain_dentry(dentry))) {
+		dentry->d_lockref.count--;
 		__dentry_kill(dentry);
 		return parent;
 	} else {