Dbuf hash table should be sized as is the arc hash table

Commit 49ddb315066e372f31bda29a5c546a9eccc8b418 added the
zfs_arc_average_blocksize parameter to allow control over the size of
the arc hash table. The dbuf hash table's size should be determined
similarly.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3721