Werk #261: df_netapp32 / df_netapp: Fixed bug with negative size in check output

The df_netapp and df_netapp32 checks tried to determine 64 bit counters by checking the existance of the oid .1.3.6.1.4.1.789.1.5.4.1.29.1 (df64TotalKBytes). Depending on the 64 Bit counter a df_netapp check is created, otherwise df_netapp32 is used. Unfortunately not all netapp interfaces start at .1, so some netapp devices where wrongly inventorized as df_netapp32 checks.
Those checks reported negative volume sizes whenever the 32 bit counter overflowed.

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