IOMMU: make the pcidevs_lock a recursive one

IOMMU: make the pcidevs_lock a recursive one

The pcidevs_lock is going to be recursively taken for hiding ATS
device, when VT-d Device-TLB flush timed out.

Signed-off-by: Quan Xu <quan.xu@intel.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Dario Faggioli <dario.faggioli@citrix.com>
pcidevs_is_locked() -> pcidevs_locked(). Add __must_check to
pcidevs_locked() and pcidevs_trylock() declarations.

Signed-off-by: Jan Beulich <jbeulich@suse.com>