zfs_arc_num_sublists_per_state should be common to all multilists

The global tunable zfs_arc_num_sublists_per_state is used by the ARC and
the dbuf cache, and other users are planned. We should change this
tunable to be common to all multilists.  This tuning may be overridden
on a per-multilist basis.

Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #5764

module/zfs/dbuf.c

@@ -673,7 +673,6 @@ retry:
 
 	multilist_create(&dbuf_cache, sizeof (dmu_buf_impl_t),
 	    offsetof(dmu_buf_impl_t, db_cache_link),
-	    zfs_arc_num_sublists_per_state,
 	    dbuf_cache_multilist_index_func);
 	refcount_create(&dbuf_cache_size);