mm/mprotect: minor can_change_pte_writable() cleanups

We want to replicate this code for handling PMDs soon.

(1) No need to crash the kernel, warning and rejecting is good enough. As
    this will no longer get optimized out, drop the pte_write() check: no
    harm would be done.

(2) Add a comment why PROT_NONE mapped pages are excluded.

(3) Add a comment regarding MAP_SHARED handling and why we rely on the
    dirty bit in the PTE.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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