fs/proc/kcore: reinstate bounce buffer for KCORE_TEXT regions

Some architectures do not populate the entire range categorised by
KCORE_TEXT, so we must ensure that the kernel address we read from is
valid.

Unfortunately there is no solution currently available to do so with a
purely iterator solution so reinstate the bounce buffer in this instance
so we can use copy_from_kernel_nofault() in order to avoid page faults
when regions are unmapped.

This change partly reverts commit 2e1c0170771e ("fs/proc/kcore: avoid
bounce buffer for ktext data"), reinstating the bounce buffer, but adapts
the code to continue to use an iterator.

[[email protected]: correct comment to be strictly correct about reasoning]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 2e1c0170771e ("fs/proc/kcore: avoid bounce buffer for ktext data")
Signed-off-by: Lorenzo Stoakes <[email protected]>
Reported-by: Jiri Olsa <[email protected]>
Closes: https://lore.kernel.org/all/ZHc2fm+9daF6cgCE@krava
Tested-by: Jiri Olsa <[email protected]>
Tested-by: Will Deacon <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: Baoquan He <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Liu Shixin <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Thorsten Leemhuis <[email protected]>
Cc: Uladzislau Rezki (Sony) <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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