audit_tree: Use mark flags to check whether mark is alive

Currently audit code uses checking of mark->inode to verify whether mark
is still alive. Switch that to checking mark flags as that is more
logical and current way will become unreliable in future.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>