nfs: only show Posix ACLs in listxattr if actually present

commit 74adf83f upstream.

The big ACL switched nfs to use generic_listxattr, which calls all existing
->list handlers.  Add a custom .listxattr implementation that only lists
the ACLs if they actually are present on the given inode.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reported-by: Philippe Troin <phil@fifi.org>
Tested-by: Philippe Troin <phil@fifi.org>
Fixes: 013cdf10 (nfs: use generic posix ACL infrastructure ...)
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>