mm, hmm: use devm semantics for hmm_devmem_{add, remove}

mm, hmm: use devm semantics for hmm_devmem_{add, remove}

devm semantics arrange for resources to be torn down when
device-driver-probe fails or when device-driver-release completes.
Similar to devm_memremap_pages() there is no need to support an explicit
remove operation when the users properly adhere to devm semantics.

Note that devm_kzalloc() automatically handles allocating node-local
memory.

Link: http://lkml.kernel.org/r/154275559545.76910.9186690723515469051.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Jérôme Glisse <[email protected]>
Cc: "Jérôme Glisse" <[email protected]>
Cc: Logan Gunthorpe <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>