[zfs-2.2.8] GCC: Fix array subscript check

Fix a zfs-2.2.8 GCC check on Fedora 42:

module/zfs/ddt.c:445:9: warning: array subscript -1 is below array
bounds of 'ddt_stat_t[64]' {aka 'struct ddt_stat[64]'} [-Warray-bounds=]

445 |         ddt_stat_add(&ddh->ddh_stat[bucket], &dds, neg);

Signed-off-by: Tony Hutter <hutter2@llnl.gov>