fs/proc/kcore.c: allow translation of physical memory addresses

When /proc/kcore is read an attempt to read the first two pages results in
HW-specific page swap on s390 and another (so called prefix) pages are
accessed instead.  That leads to a wrong read.

Allow architecture-specific translation of memory addresses using
kc_xlate_dev_mem_ptr() and kc_unxlate_dev_mem_ptr() callbacks similarily
to /dev/mem xlate_dev_mem_ptr() and unxlate_dev_mem_ptr() callbacks.  That
way an architecture can deal with specific physical memory ranges.

Re-use the existing /dev/mem callback implementation on s390, which
handles the described prefix pages swapping correctly.

For other architectures the default callback is basically NOP.  It is
expected the condition (vaddr == __va(__pa(vaddr))) always holds true for
KCORE_RAM memory type.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Gordeev <[email protected]>
Suggested-by: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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