UBIFS: make space fixup work in the remount case

The UBIFS space fixup is a useful feature which allows to fixup the "broken"
flash space at the time of the first mount. The "broken" space is usually the
result of using a "dumb" industrial flasher which is not able to skip empty
NAND pages and just writes all 0xFFs to the empty space, which has grave
side-effects for UBIFS when UBIFS trise to write useful data to those empty
pages.

The fix-up feature works roughly like this:
1. mkfs.ubifs sets the fixup flag in UBIFS superblock when creating the image
   (see -F option)
2. when the file-system is mounted for the first time, UBIFS notices the fixup
   flag and re-writes the entire media atomically, which may take really a lot
   of time.
3. UBIFS clears the fixup flag in the superblock.

This works fine when the file system is mounted R/W for the very first time.
But it did not really work in the case when we first mount the file-system R/O,
and then re-mount R/W. The reason was that we started the fixup procedure too
late, which we cannot really do because we have to fixup the space before it
starts being used.
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Reported-by: Mark Jackson <mpfj-list@mimc.co.uk>
Cc: stable@vger.kernel.org # 3.0+

fs/ubifs/super.c

@@ -1568,6 +1568,12 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 	c->remounting_rw = 1;
 	c->ro_mount = 0;
 
+	if (c->space_fixup) {
+		err = ubifs_fixup_free_space(c);
+		if (err)
+			return err;
+	}
+
 	err = check_free_space(c);
 	if (err)
 		goto out;
@@ -1684,12 +1690,6 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 		err = dbg_check_space_info(c);
 	}
 
-	if (c->space_fixup) {
-		err = ubifs_fixup_free_space(c);
-		if (err)
-			goto out;
-	}
-
 	mutex_unlock(&c->umount_mutex);
 	return err;