mm/memory.c: do_numa_page(): remove a redundant page table read

do_numa_page() is reading from the same page table entry, twice, while
holding the page table lock: once while checking that the pte hasn't
changed, and again in order to modify the pte.

Instead, just read the pte once, and save it in the same old_pte variable
that already exists.  This has no effect on behavior, other than to
provide a tiny potential improvement to performance, by avoiding the
redundant memory read (which the compiler cannot elide, due to
READ_ONCE()).

Also improve the associated comments nearby.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: John Hubbard <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Reviewed-by: Ryan Roberts <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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