Automatically call Inode::flush_metadata() before an Inode is destroyed.

Use a little template magic to have Retainable::release() call out to
T::will_be_destroyed() if such a function exists before actually calling
the destructor. This gives us full access to virtual functions in the
pre-destruction code.

Kernel/Process.cpp

@@ -1104,7 +1104,7 @@ int Process::sys$utime(const char* pathname, const Unix::utimbuf* buf)
         atime = now;
     }
     inode.set_atime(atime);
-    inode.set_mtime(atime);
+    inode.set_mtime(mtime);
     inode.flush_metadata();
     return 0;
 }