fix debugfs inode leak

Looking at the reiser4 crash, I found a leak in debugfs. In
debugfs_mknod(), we create the inode before checking if the dentry
already has one attached. We don't free it if that is the case.

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>