tools/testing/selftests/vm/hugetlb-madvise.c: silence uninitialized variable warning

tools/testing/selftests/vm/hugetlb-madvise.c: silence uninitialized variable warning

This code just reads from memory without caring about the data itself.
However static checkers complain that "tmp" is never properly initialized.
Initialize it to zero and change the name to "dummy" to show that we
don't care about the value stored in it.

Link: https://lkml.kernel.org/r/YtZ8mKJmktA2GaHB@kili
Fixes: c4b6cb884011 ("selftests/vm: add hugetlb madvise MADV_DONTNEED MADV_REMOVE test")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>