Avoid 64bit division in multilist index functions

The number of sublists in a multilist is relatively small. We dont need
64 bits to calculate an index. 32 bits is sufficient and makes the
code more efficient.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> 
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12288

module/zfs/metaslab.c

@@ -1874,7 +1874,12 @@ static unsigned int
 metaslab_idx_func(multilist_t *ml, void *arg)
 {
 	metaslab_t *msp = arg;
-	return (msp->ms_id % multilist_get_num_sublists(ml));
+
+	/*
+	 * ms_id values are allocated sequentially, so full 64bit
+	 * division would be a waste of time, so limit it to 32 bits.
+	 */
+	return ((unsigned int)msp->ms_id % multilist_get_num_sublists(ml));
 }
 
 uint64_t