Use fallocate() rather than truncate() when creating SHM memory

With truncate() the OS might not actually allocate the space leading to
a SIGBUS if /dev/shm runs out of space when actually using the mmap.

By using fallocate we ensure that once the SHM mmap is created it wont
fail

tools/utils/shm/fallocate_linux.go

@@ -0,0 +1,20 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package shm
+
+import (
+	"errors"
+	"fmt"
+
+	"golang.org/x/sys/unix"
+)
+
+var _ = fmt.Print
+
+func Fallocate_simple(fd int, size int64) (err error) {
+	for {
+		if err = unix.Fallocate(fd, 0, 0, size); !errors.Is(err, unix.EINTR) {
+			return
+		}
+	}
+}