jbd2: move more common code into journal_init_common()

There are some repetitive code in jbd2_journal_init_dev() and
jbd2_journal_init_inode(). So this patch moves the common code into
journal_init_common() helper to simplify the code. And fix the coding
style warnings reported by checkpatch.pl by the way.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>