mm/hmm: define the pre-processor related parts of hmm.h even if disabled

Only the function calls are stubbed out with static inlines that always
fail. This is the standard way to write a header for an optional component
and makes it easier for drivers that only optionally need HMM_MIRROR.

Link: https://lore.kernel.org/r/20191112202231.3856-5-jgg@ziepe.ca


Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>