Check that access_process_vm() is accessing a valid mapping in the target
process.

This limits ptrace() accesses and accesses through /proc/<pid>/maps to only
those regions actually mapped by a program.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>