btrfs: remove save_error_info()

Actually save_error_info() sets the FS state to error and nothing else.
Further the word save doesn't induce caffeine when compared to the word
set in what actually it does.

So to make it better understandable move save_error_info() code to its
only consumer itself.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>