d_prune_alias(): just lock the parent and call __dentry_kill()

The only reason for games with ->d_prune() was __d_drop(), which
was needed only to force dput() into killing the sucker off.

Note that lock_parent() can be called under ->i_lock and won't
drop it, so dentry is safe from somebody managing to kill it
under us - it won't happen while we are holding ->i_lock.

__dentry_kill() is called only with ->d_lockref.count being 0
(here and when picked from shrink list) or 1 (dput() and dropping
the ancestors in shrink_dentry_list()), so it will never be called
twice - the first thing it's doing is making ->d_lockref.count
negative and once that happens, nothing will increment it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -770,20 +770,13 @@ restart:
 	hlist_for_each_entry(dentry, &inode->i_dentry, d_alias) {
 		spin_lock(&dentry->d_lock);
 		if (!dentry->d_lockref.count) {
-			/*
-			 * inform the fs via d_prune that this dentry
-			 * is about to be unhashed and destroyed.
-			 */
-			if ((dentry->d_flags & DCACHE_OP_PRUNE) &&
-			    !d_unhashed(dentry))
-				dentry->d_op->d_prune(dentry);
-
-			__dget_dlock(dentry);
-			__d_drop(dentry);
-			spin_unlock(&dentry->d_lock);
-			spin_unlock(&inode->i_lock);
-			dput(dentry);
-			goto restart;
+			struct dentry *parent = lock_parent(dentry);
+			if (likely(!dentry->d_lockref.count)) {
+				__dentry_kill(dentry);
+				goto restart;
+			}
+			if (parent)
+				spin_unlock(&parent->d_lock);
 		}
 		spin_unlock(&dentry->d_lock);
 	}