Found by Al Viro.
Signed-off-by: NJoern Engel <joern@logfs.org>

rootdir was already allocated, so we must iput it again.
Found by Al Viro.
Signed-off-by: NJoern Engel <joern@logfs.org>

If the first superblock is wrong and the second gets written, there
will still be a mismatch on next mount.  Write both to make sure they
match.
Signed-off-by: NJoern Engel <joern@logfs.org>

logfs_recover_sb() needs it open.
Signed-off-by: NJoern Engel <joern@logfs.org>

This prevents unnecessary journal writes.  More importantly it prevents
an oops due to a journal write on failed mount.

Erases for block devices were always just emulated by writing 0xff.
Some time back the write was removed and only the page cache was
changed to 0xff.  Superficialy a good idea with two problems:
1. Touching the page cache isn't necessary either.
2. However, writing out 0xff _is_ necessary for the journal.  As the
   journal is scanned linearly, an old non-overwritten commit entry
   can be used on next mount and cause havoc.

This should fix both aspects.

This is a new flash file system. See
Documentation/filesystems/logfs.txt
Signed-off-by: NJoern Engel <joern@logfs.org>