ext4: unify the ext4 super block loading operation

ext4: unify the ext4 super block loading operation

Now we load the super block from the disk in two steps. First we load
the super block with the default block size(EXT4_MIN_BLOCK_SIZE). Second
we load the super block with the real block size. The second step is a
little far from the first step. This patch move these two steps together
in a new function.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220916141527.1012715-15-yanaijie@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>