aoe: do not BUG if memory pressure prevented debugfs file creation

If the system has trouble allocating memory for the creation of the aoe
debugfs directory or of a file inside it, the debugfs member of an aoedev
can be NULL.

Do not treat a NULL debugfs pointer as a BUG on aoedev shutdown, avoiding
the user impact of an unecessary panic.

Signed-off-by: Ed Cashin <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/block/aoe/aoeblk.c

···
 void
 aoedisk_rm_debugfs(struct aoedev *d)
 {
-	BUG_ON(d->debugfs == NULL);
 	debugfs_remove(d->debugfs);
 	d->debugfs = NULL;
 }