Add cpus_allowed option

cpumask can be cumbersome to use, since it requires you to calculate
a decimal mask to pass to fio. So add a cpus_allowed option that
takes a text list of allowed cpus, ala: cpus_allowed=1,3,8 will set
the affinity mask for CPUS 1, 3, and 8.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 7e706ab..0c24307 100644
--- a/HOWTO
+++ b/HOWTO
@@ -472,6 +472,10 @@
 		sched_setaffinity(2). This may not work on all supported
 		operating systems or kernel versions.
 
+cpus_allowed=str Controls the same options as cpumask, but it allows a text
+		setting of the permitted CPUs instead. So to use CPUs 1 and
+		5, you would specify cpus_allowed=1,5.
+
 startdelay=int	Start this job the specified number of seconds after fio
 		has started. Only useful if the job file contains several
 		jobs, and you want to delay starting some jobs to a certain