Allow percentage setting for size=

Sometimes it's useful to set this to a particular size of a device.
Allowing percentages makes this more easy, as one does not have to
do the math outside of fio and pass in as environment variables
or custom job files.

To use, simply add a

	size=15%

or whatever is needed, then fio will use 15% of the total size of
the device(s) or file(s) given. Percentage can be from 1 to 100.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/HOWTO b/HOWTO
index ee899b8..ce1bd9e 100644
--- a/HOWTO
+++ b/HOWTO
@@ -382,7 +382,10 @@
 		fio will divide this size between the available files
 		specified by the job. If not set, fio will use the full
 		size of the given files or devices. If the the files
-		do not exist, size must be given.
+		do not exist, size must be given. It is also possible to
+		give size as a percentage between 1 and 100. If size=20%
+		is given, fio will use 20% of the full size of the given
+		files or devices.
 
 filesize=int	Individual file sizes. May be a range, in which case fio
 		will select sizes for files at random within the given range