Update snmp__df.in

Handle large partitions.

hrStorageSize and hrStorageUsed might "wrap around". 
Example:
pj_.value 211.998954212829
pi_.value -13.1452642689504

Both values are bonkers.

By adding the patch, I get this:
pj_.value 68.6950857122177
pi_.value 7.88728932007461

Which is what the correct percentage usage is.