um: Use asm-generic/dma-mapping.h

If DMA (PCI over virtio) is enabled, then some drivers may
enable CONFIG_DMA_OPS as well, and then we pull in the x86
definition of get_arch_dma_ops(), which uses the dma_ops
symbol, which isn't defined.

Since we don't have real DMA ops nor any kind of IOMMU fix
this in the simplest possible way: pull in the asm-generic
file instead of inheriting the x86 one. It's not clear why
those drivers that do (e.g. VDPA) "select DMA_OPS", and if
they'd even work with this, but chances are nobody will be
wanting to do that anyway, so fixing the build failure is
good enough.

Reported-by: Randy Dunlap <[e-mail]>
Fixes: 68f5d3f3b654 ("um: add PCI over virtio emulation driver")
Signed-off-by: Johannes Berg <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Cc: [e-mail]
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/include/asm/Kbuild

···
 generic-y += compat.h
 generic-y += current.h
 generic-y += device.h
+generic-y += dma-mapping.h
 generic-y += emergency-restart.h
 generic-y += exec.h
 generic-y += extable.h