jbd2: factor out journal initialization from journal_get_superblock()

Current journal_get_superblock() couple journal superblock checking and
partial journal initialization, factor out initialization part from it
to make things clear.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230315013128.3911115-6-chengzhihao1@huawei.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>