iommufd: Fix uninitialized rc in iommufd_access_rw()

iommufd: Fix uninitialized rc in iommufd_access_rw()

Reported by smatch:
drivers/iommu/iommufd/device.c:1392 iommufd_access_rw() error: uninitialized symbol 'rc'.

Fixes: 8d40205f6093 ("iommufd: Add kAPI toward external drivers for kernel access")
Link: https://patch.msgid.link/r/20250227200729.85030-1-nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Closes: https://lore.kernel.org/r/202502271339.a2nWr9UA-lkp@intel.com/
[nicolinc: can't find an original report but only in "old smatch warnings"]
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>