selftests/vm: write strlen length instead of sizeof to nr_hugepages

When setting back the initial value to nr_hugepages, the
test was writing a length sizeof of the string and checking
that strlen was writen. Since those values are not the same,
use strlen in both place instead.

Also make the error messages more explicit to help in future
debugging.

Signed-off-by: Yannick Brosseau <scientist@fb.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>