ext4: cleanup ext4_get_dev_journal() and ext4_get_journal()

Factor out a new helper form ext4_get_dev_journal() to get external
journal bdev and check validation of this device, drop ext4_blkdev_get()
helper, and also remove duplicate check of journal feature. It makes
ext4_get_dev_journal() more clear than before.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230811063610.2980059-12-yi.zhang@huaweicloud.com


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