unix: adjust statfs_freebsd11_t member types on freebsd/arm

Follow-up for CL 239060.

Otherwise the types don't match when copy()'ing in (*Statfs_t).convertFrom

Change-Id: I8b05247d01e1ac120ea378806793dc1c996f9a4b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/239397
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/ztypes_freebsd_arm.go

@@ -150,9 +150,9 @@ type statfs_freebsd11_t struct {
 	Owner       uint32
 	Fsid        Fsid
 	Charspare   [80]int8
-	Fstypename  [16]int8
-	Mntfromname [88]int8
-	Mntonname   [88]int8
+	Fstypename  [16]byte
+	Mntfromname [88]byte
+	Mntonname   [88]byte
 }
 
 type Flock_t struct {