mount: enable fuse readahead

Apparently readahead was disabled by default. Enable readahead with the
Linux default size of 128kB. Larger values seem to have no effect.
This can speed up reading from the fuse mount by at least factor 5.

Speedup for a 1G random file stored in a local repository:
(Only one result shown, but times were quite stable, restarted restic
after each command)
$ dd if=/dev/urandom bs=1M count=1024 of=rand
$ shasum -a 256 tmp/rand
75dd9b374e712577d64672a05b8ceee40dfc45dce6321082d2c2fd51d60c6c2d  tmp/rand

before: $ time shasum -a 256 fuse/snapshots/latest/tmp/rand
75dd9b374e712577d64672a05b8ceee40dfc45dce6321082d2c2fd51d60c6c2d  fuse/snapshots/latest/tmp/rand

real    0m18.294s
user    0m4.522s
sys     0m3.305s

before: $ time cat fuse/snapshots/latest/tmp/rand > /dev/null
real    0m14.924s
user    0m0.000s
sys     0m4.625s

after:  $ time shasum -a 256 fuse/snapshots/latest/tmp/rand
75dd9b374e712577d64672a05b8ceee40dfc45dce6321082d2c2fd51d60c6c2d  fuse/snapshots/latest/tmp/rand

real    0m6.106s
user    0m3.115s
sys     0m0.182s

after:  $ time cat fuse/snapshots/latest/tmp/rand > /dev/null
real    0m3.096s
user    0m0.017s
sys     0m0.241s

cmd/restic/cmd_mount.go

@@ -122,6 +122,7 @@ func runMount(opts MountOptions, gopts GlobalOptions, args []string) error {
 	mountOptions := []systemFuse.MountOption{
 		systemFuse.ReadOnly(),
 		systemFuse.FSName("restic"),
+		systemFuse.MaxReadahead(128 * 1024),
 	}
 
 	if opts.AllowOther {