iommupt: Return ERR_PTR from _table_alloc()

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: aefd967dab64 ("iommupt: Use the incoherent start/stop functions for PT_FEAT_DMA_INCOHERENT")
Fixes: dcd6a011a8d5 ("iommupt: Add map_pages op")
Fixes: cdb39d918579 ("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>