staging: lustre: ldlm: Fix extern variable declaration

Move the extern declaration to a header file common to all users of the
variable. This fixes the following sparse warning:
symbol 'ldlm_cancel_unused_locks_before_replay' was not declared.
Should it be static?

Signed-off-by: Imre Deak <imre.deak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>