pktcdvd: debugfs functions return NULL on error

My static checker complains correctly that this is potential NULL
dereference because debugfs functions return NULL on error.  They return
an ERR_PTR if they are configured out.

We don't need to check for ERR_PTR because if debugfs is stubbed out the
dummy functions won't complain about that.  We don't need to check the
values before calling debugfs_remove() because that accepts ERR_PTRs and
NULL pointers.

We don't need to set pkt->dfs_f_info to NULL in pkt_debugfs_dev_new()
because it was initialized with kzalloc() so I have removed that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>