aoe: create and destroy debugfs directory for aoe

This series adds the debugging information that the coraid.com-distributed
aoe driver exports via sysfs, but instead of sysfs, it uses debugfs.

With these patches applied, even without AoE targets on the network, KEDR
reports new possible memory leaks, but these are from callers outside the
aoe driver that have used aoe_devnode to get the name of the character
devices through the aoe_class->devnode callback, and I believe they're
responsible for freeing that memory.

This patch:

Create and destroy the debugfs directory.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>