iommupt: Return ERR_PTR from _table_alloc()

syzkaller noticed that with fault injection a failure inside
iommu_alloc_pages_node_sz() oops's in PT_FEAT_DMA_INCOHERENT because it goes
on to make NULL incoherent. Closer inspection shows the return value has
become confused, the alloc routines on the iommupt side expect ERR_PTR while
iommu_alloc_pages_node_sz() returns NULL.

Error out early to fix both issues.

Fixes: aefd967 ("iommupt: Use the incoherent start/stop functions for PT_FEAT_DMA_INCOHERENT")
Fixes: dcd6a01 ("iommupt: Add map_pages op")
Fixes: cdb39d9 ("iommupt: Add the basic structure of the iommu implementation")
Reported-by: syzbot+e06bb7478e687f235ad7@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/693a39de.050a0220.4004e.02ce.GAE@google.com/
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

Author: jgunthorpe

drivers/iommu/generic_pt/iommu_pt.h

@@ -372,6 +372,9 @@ static inline struct pt_table_p *_table_alloc(struct pt_common *common,
 
 	table_mem = iommu_alloc_pages_node_sz(iommu_table->nid, gfp,
 					      log2_to_int(lg2sz));
+	if (!table_mem)
+		return ERR_PTR(-ENOMEM);
+
 	if (pt_feature(common, PT_FEAT_DMA_INCOHERENT) &&
 	    mode == ALLOC_NORMAL) {
 		int ret = iommu_pages_start_incoherent(