man mke2fs:
-m reserved-blocks-percentage
Specify the percentage of the filesystem blocks reserved
or the super-user. This avoids fragmentation, and allows
root-owned daemons, such as syslogd(8), to continue to function
correctly after non-privileged processes are prevented
from wriiting to the filesystem. The default percentage is 5%.
убрать, соответственно, можно с tune2fs