checkpatch.pl: Suggest lockdep instead of asserting !spin_is_locked()

This commit points people who might otherwise code up something like
WARN_ON(!spin_is_locked(&mylock)) to lockdep_assert_held(&mylock).

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>