Add option for specifically setting buffer contents

Fio can use zeroes, slightly scrambled data, full random data,
or specifically compressible data. With this option, the user
can now fully control the pattern written, similarly to how
verify_pattern works for verify=meta.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/HOWTO b/HOWTO
index 2183be1..c3672d8 100644
--- a/HOWTO
+++ b/HOWTO
@@ -547,6 +547,11 @@
 		alternate random and zeroed data throughout the IO
 		buffer.
 
+buffer_pattern=str	If set, fio will fill the io buffers with this pattern.
+		If not set, the contents of io buffers is defined by the other
+		options related to buffer contents. The setting can be any
+		pattern of bytes, and can be prefixed with 0x for hex values.
+
 nrfiles=int	Number of files to use for this job. Defaults to 1.
 
 openfiles=int	Number of files to keep open at the same time. Defaults to