[PATCH] IPoIB: document conversion to debugfs

Update IPoIB documentation now that multicast debugging files have moved from
ipoibdebugfs to debugfs.

Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>