xfs: fix spin_is_locked assert on uni-processor builds

Without SMP or preemption spin_is_locked always returns false,
so we can't do an assert with it.  Instead use assert_spin_locked,
which does the right thing on all builds.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reported-by: Johannes Engel <jcnengel@googlemail.com>
Tested-by: Johannes Engel <jcnengel@googlemail.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>