Spurious ENOMEM returns when reading dbufs kstat

Commit 7b2d78a046aa4695d434478a439a9438521d73af fixed some improper uses
of snprintf(), however, in __dbuf_stats_hash_table_data() the return
value of snprintf is propagated to the caller. This caused spurious
ENOMEM errors when reading the dbufs kstat.

This commit causes the actual number of characters written to be returned.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3072