mm/hmm: remove HMM_PFN_SPECIAL

This is just an alias for HMM_PFN_ERROR, nothing cares that the error was
because of a special page vs any other error case.

Link: https://lore.kernel.org/r/4-v2-b4e84f444c7d+24f57-hmm_no_flags_jgg@mellanox.com


Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>