fs/proc/task_mmu.c: do not show VmExe bigger than total executable virtual memory

If start_code / end_code pointers are screwed then "VmExe" could be
bigger than total executable virtual memory and "VmLib" becomes
negative:

  VmExe:	  294320 kB
  VmLib:	18446744073709327564 kB

VmExe and VmLib documented as text segment and shared library code size.

Now their sum will be always equal to mm->exec_vm which sums size of
executable and not writable and not stack areas.

I've seen this for huge (>2Gb) statically linked binary which has whole
world inside.  For it start_code ..  end_code range also covers one of
rodata sections.  Probably this is bug in customized linker, elf loader
or both.

Anyway CONFIG_CHECKPOINT_RESTORE allows to change these pointers, thus
we cannot trust them without validation.

Link: http://lkml.kernel.org/r/150728955451.743749.11276392315459539583.stgit@buzz


Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>