selftest/vm: add util.h and and move helper functions there

Avoid code duplication by adding util.h.  No functional change in this
patch.

Link: https://lkml.kernel.org/r/20220307054355.149820-1-aneesh.kumar@linux.ibm.com


Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>