kselftest: fix OOM in memory compaction test

Running the compaction_test sometimes results in out-of-memory
failures. When I debugged this, it turned out that the code to
reset the number of hugepages to the initial value is simply
broken since we write into an open sysctl file descriptor
multiple times without seeking back to the start.

Adding the lseek here fixes the problem.

Cc: stable@vger.kernel.org
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Link: https://bugs.linaro.org/show_bug.cgi?id=3145


Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>