oprofile, s390: Fix uninitialized memory access when writing to oprofilefs

If oprofilefs_ulong_from_user() is called with count equals zero, *val
remains unchanged. Depending on the implementation it might be
uninitialized. Fixing users of oprofilefs_ulong_ from_user().

We missed these s390 changes with:

 913050b oprofile: Fix uninitialized memory access when writing to writing to oprofilefs

Cc: stable@vger.kernel.org  # 3.3+
Signed-off-by: Robert Richter <robert.richter@amd.com>