Impact: introduce new lockdep API

Allow to change a held lock's class. Basically the same as the existing
code to change a subclass therefore reuse all that.

The XFS code will be able to use this to annotate their inode locking.
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: NIngo Molnar <mingo@elte.hu>