lockdep: Make lockdep checking constant

There are several places in the kernel which would like to pass a const
pointer to lockdep_is_held().  Constify the entire path so nobody has to
trick the compiler.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Link: https://lkml.kernel.org/r/20180117151414.23686-3-willy@infradead.org