vfio/pds: fix return value in pds_vfio_get_lm_file()

anon_inode_getfile() never returns NULL pointer, it will return
ERR_PTR() when it fails, so replace the check with IS_ERR().

Fixes: bb500dbe ("vfio/pds: Add VFIO live migration support")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Link: https://lore.kernel.org/r/20230819023716.3469037-1-yangyingliang@huawei.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>