ext4: factor out ext4_hash_info_init()

Factor out ext4_hash_info_init() to simplify __ext4_fill_super(). No
functional change.

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20230323140517.1070239-2-yanaijie@huawei.com


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