Change the pcidevs_lock from rw_lock to spin_lock

Change the pcidevs_lock from rw_lock to spin_lock

As pcidevs_lock is changed from protecting only the alldevs_list to
more than that, it doesn't benifit too much from the rw_lock. Also the
previous patch 18906:2941b1a97c60 is wrong to use read_lock to protect some
sensitive data (thanks Espen pointed out that).

Also two minor fix in this patch:
a) deassign_device will deadlock when try to get the pcidevs_lock if
called by pci_release_devices, remove the lock to the caller.
b) The iommu_domain_teardown should not ASSERT for the pcidevs_lock
because it just update the domain's vt-d mapping.

Signed-off-by: Yunhong Jiang <yunhong.jiang@intel.com>