fs/adfs: dir: improve update failure handling

When we update a directory, a number of errors may happen. If we failed
to find the entry to update, we can just release the directory buffers
as normal.

However, if we have some other error, we may have partially updated the
buffers, resulting in an invalid directory. In this case, we need to
discard the buffers to avoid writing the contents back to the media, and
later re-read the directory from the media.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>