vfs: add --vfs-read-ahead parameter for use with --vfs-cache-mode full

This parameter causes extra read-ahead over --buffer-size which is not
buffered in memory but on disk.

vfs/vfscommon/options.go

@@ -31,6 +31,7 @@ type Options struct {
 	WriteWait         time.Duration // time to wait for in-sequence write
 	ReadWait          time.Duration // time to wait for in-sequence read
 	WriteBack         time.Duration // time to wait before writing back dirty files
+	ReadAhead         fs.SizeSuffix // bytes to read ahead in cache mode "full"
 }
 
 // DefaultOpt is the default values uses for Opt
@@ -56,4 +57,5 @@ var DefaultOpt = Options{
 	WriteWait:         1000 * time.Millisecond,
 	ReadWait:          20 * time.Millisecond,
 	WriteBack:         5 * time.Second,
+	ReadAhead:         0 * fs.MebiByte,
 }