VFS: reimplement truncate mtime/ctime fix

POSIX mandates that a file's modification and change time be left
untouched upon truncate/ftruncate iff the file size does not change.
However, an open(O_TRUNC) call must always update the modification and
change time of the file, even if it was already zero-sized. VFS uses
the file systems' truncate call to implement O_TRUNC. This patch
replaces git-255ae85, which did not take into account the open case.
The size check is now moved into VFS, so that individual file systems
need not check for this case anymore.

servers/ext2/link.c

@@ -563,8 +563,6 @@ off_t newsize;			/* inode must become this size */
 	return(EINVAL);
   if (newsize > rip->i_sp->s_max_size)	/* don't let inode grow too big */
 	return(EFBIG);
-  if (rip->i_size == newsize)
-	return(OK);
 
   /* Free the actual space if truncating. */
   if (newsize < rip->i_size) {