lib: replace kmap() with kmap_local_page()

kmap() has been deprecated in favor of the kmap_local_page() due to high
cost, restricted mapping space, the overhead of a global lock for
synchronization, and making the process sleep in the absence of free
slots.

kmap_local_page() is faster than kmap() and offers thread-local and
CPU-local mappings, take pagefaults in a local kmap region and preserves
preemption by saving the mappings of outgoing tasks and restoring those of
the incoming one during a context switch.

The mappings are kept thread local in the functions “dmirror_do_read”
and “dmirror_do_write” in test_hmm.c

Therefore, replace kmap() with kmap_local_page() and use
mempcy_from/to_page() to avoid open coding kmap_local_page() + memcpy() +
kunmap_local().

Remove the unused variable “tmp”.

Link: https://lkml.kernel.org/r/20230610175712.GA348514@sumitra.com
Signed-off-by: Sumitra Sharma <sumitraartsy@gmail.com>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: Deepak R Varma <drv@mailo.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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