configfs work

A couple of fixes (UAF in configfs_lookup() and really old races
introduced when lseek() on configfs directories stopped locking those
directories; impact up to and including UAF).

Fixes aside, the main result is that configfs is finally switched
to tree-in-dcache machinery.  It's *not* making use of recursive removal
helpers yet, and it still does the bloody awful "build subtree in full
sight of userland, with possibility of failure halfway through and need
to unroll" that forces the locking model from hell; dealing with that
is a separate patch series, once this one is out of the way.  However,
it is using DCACHE_PERSISTENT properly now.  And apparmorfs is the sole
remaining user of __simple_{unlink,rmdir}() at that point.

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

create_default_group(): pass parent's dentry instead of config_group

the only way parent_group is used there...

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>