x86/PCI: only check for spinlock being held in SMP kernels

spin_is_locked() is always false on UP kernels: spin_lock_irqsave() does no
locking, so we can't tell whether the lock is held or not.  Therefore,
this warning is only valid for SMP kernels.

CC: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>