Fix zdb printf format string for ZIL data blocks

Without this fix the zdb printouts of ZIL data blocks look full of FF
due to printf() handling its arguments as int by default.

Here is the output before the fix

TX_WRITE            len   4136, txg 1093817, seq 149231
        foid 4242, offset 0, length f68
        G FFFFFF8EFFFFFF87FFFFFF91FFFFFFCC 1c

FFFFFFAFFFFFFFC9FFFFFFBAZ FFFFFFC3

And the same after the fix

TX_WRITE            len   4136, txg 1093817, seq 149231
        foid 4242, offset 0, length f68
        G 8E8791CC 1cAFC9BAZ C3

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #962