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: c4b6cb88 ("selftests/vm: add hugetlb madvise MADV_DONTNEED MADV_REMOVE test")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Souptick Joarder (HPE) <jrdr.linux@gmail.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>