vfs: fix reference leak in d_prune_aliases()

In "d_prune_alias(): just lock the parent and call __dentry_kill()" the old
dget + d_drop + dput has been replaced with lock_parent + __dentry_kill;
unfortunately, dput() does more than just killing dentry - it also drops the
reference to parent.  New variant leaks that reference and needs dput(parent)
after killing the child off.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -778,6 +778,7 @@ restart:
 			struct dentry *parent = lock_parent(dentry);
 			if (likely(!dentry->d_lockref.count)) {
 				__dentry_kill(dentry);
+				dput(parent);
 				goto restart;
 			}
 			if (parent)