Fix how we check zram stats from /sys/block/zram0/mm_stat
Fix how we check zram stats from /sys/block/zram0/mm_stat

There is one new stat since 4.18. Checking if we read more than
7 values should be enough to be sure that the file has the stats
we want.

rhinstaller/libblockdev,rhinstaller/libblockdev,rhinstaller/libblockdev