lib/list_sort: test: check element addresses

Improve 'lib_sort()' test and check that:
 o 'cmp()' is called only for elements which were present in the original list,
   i.e., the 'a' and 'b' parameters are valid
 o the resulted (sorted) list consists onlly of the original elements
 o intdoruce "poison" fields to make sure data around 'struc list_head' field
   are not corrupted.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: Don Mullis <don.mullis@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>