jbd2: cleanup load_superblock()

jbd2: cleanup load_superblock()

Rename load_superblock() to journal_load_superblock(), move getting and
reading superblock from journal_init_common() and
journal_get_superblock() to this function, and also rename
journal_get_superblock() to journal_check_superblock(), make it a pure
check helper to check superblock validity from disk.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230811063610.2980059-7-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>