ext4: factor out ext4_init_metadata_csum()

Factor out ext4_init_metadata_csum(). No functional change.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://lore.kernel.org/r/20220916141527.1012715-9-yanaijie@huawei.com


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