LU-4199 libcfs: remove assertion of spin_is_locked()

LU-4199 libcfs: 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>
Change-Id: Id8a7207b6fb08f10ccd4447263fec805df31ed67
Reviewed-on: http://review.whamcloud.com/8144
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>