jbd2: split updating of journal superblock and marking journal empty

jbd2: split updating of journal superblock and marking journal empty

commit 24bcc89c7e7c64982e6192b4952a0a92379fc341 upstream.

There are three case of updating journal superblock. In the first case, we want
to mark journal as empty (setting s_sequence to 0), in the second case we want
to update log tail, in the third case we want to update s_errno. Split these
cases into separate functions. It makes the code slightly more straightforward
and later patches will make the distinction even more important.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
[bwh: Prerequisite for "jbd2: fix ocfs2 corrupt when updating journal
 superblock fails".
 Backported to 3.2: drop changes to trace events.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>