Fix various typos

Correct an assortment of typos throughout the code base.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #11774

module/zfs/arc.c

@@ -5036,7 +5036,7 @@ arc_reap_cb(void *arg, zthr_t *zthr)
 	 * memory in the system at a fraction of the arc_size (1/128th by
 	 * default).  If oversubscribed (free_memory < 0) then reduce the
 	 * target arc_size by the deficit amount plus the fractional
-	 * amount.  If free memory is positive but less then the fractional
+	 * amount.  If free memory is positive but less than the fractional
 	 * amount, reduce by what is needed to hit the fractional amount.
 	 */
 	free_memory = arc_available_memory();