vfio/spapr: Fix missing mutex unlock when creating a window

[ Upstream commit 2da64d20a0b20046d688e44f4033efd09157e29d ]

Commit d9c728949ddc ("vfio/spapr: Postpone default window creation")
added an additional exit to the VFIO_IOMMU_SPAPR_TCE_CREATE case and
made it possible to return from tce_iommu_ioctl() without unlocking
container->lock; this fixes the issue.

Fixes: d9c728949ddc ("vfio/spapr: Postpone default window creation")
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>