ext4: factor out ext4_block_group_meta_init()

Factor out ext4_block_group_meta_init(). No functional change.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20230323140517.1070239-8-yanaijie@huawei.com


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