Kernel: Added unmount ability to VFS

It is now possible to unmount file systems from the VFS via `umount`.
It works via looking up the `fsid` of the filesystem from the `Inode`'s
metatdata so I'm not sure how fragile it is. It seems to work for now
though as something to get us going.

Userland/umount.cpp

@@ -0,0 +1,22 @@
+#include <LibCore/CArgsParser.h>
+#include <stdio.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    CArgsParser args_parser("umount");
+    args_parser.add_arg("mountpoint", "mount point");
+    CArgsParserResult args = args_parser.parse(argc, argv);
+
+    if (argc == 2) {
+        if (umount(argv[1]) < 0) {
+            perror("umount");
+            return 1;
+        }
+    } else {
+        args_parser.print_usage();
+        return 0;
+    }
+
+    return 0;
+}