UBIFS: add a comment in key.h for unused parameter

Add a comment in key.h to explain why we keep an unused
parameter in key helpers.

CRs-Fixed: 975289
Change-Id: I88633f34f7f49b5b895f2295c1ce4ad250900fc0
Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git-commit: 170eb55f7d4ba9564736ba298a7d4985422db4cc
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>