staging/lustre: remove assertion of spin_is_locked()

spin_is_locked() is always false when the platform is
uniprocessor and CONFIG_DEBUG_SPINLOCK is not enabled.
This patch replaces its assertion by assert_spin_locked().

Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Reviewed-on: http://review.whamcloud.com/8144
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4199
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>