Fix "constant 2435016766 overflows int32" build failure on arm

Cast to uint32 to fix the int32 overflow, and define BTRFS_SUPER_MAGIC
locally to fix the next failure on darwin.

mount.go

@@ -100,9 +100,12 @@ func doMount(args *argContainer) {
 	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
 	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
 	if !args.noprealloc {
+		// darwin does not have unix.BTRFS_SUPER_MAGIC, so we define it here
+		const BTRFS_SUPER_MAGIC = 0x9123683e
 		var st unix.Statfs_t
 		err = unix.Statfs(args.cipherdir, &st)
-		if err == nil && st.Type == unix.BTRFS_SUPER_MAGIC {
+		// Cast to uint32 avoids compile error on arm: "constant 2435016766 overflows int32"
+		if err == nil && uint32(st.Type) == BTRFS_SUPER_MAGIC {
 			tlog.Info.Printf(tlog.ColorYellow +
 				"Btrfs detected, forcing -noprealloc. See https://github.com/rfjakob/gocryptfs/issues/395 for why." +
 				tlog.ColorReset)