mm/memory: return vm_fault_t result from migrate_to_ram() callback

The migrate_to_ram() callback should always succeed, but in rare cases can
fail usually returning VM_FAULT_SIGBUS.  Commit 16ce101d
("mm/memory.c: fix race when faulting a device private page") incorrectly
stopped passing the return code up the stack.  Fix this by setting the ret
variable, restoring the previous behaviour on migrate_to_ram() failure.

Link: https://lkml.kernel.org/r/20221114115537.727371-1-apopple@nvidia.com
Fixes: 16ce101d

 ("mm/memory.c: fix race when faulting a device private page")
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>