ext4: factor out __ext4_mb_scan_group()

Extract __ext4_mb_scan_group() to make the code clearer and to
prepare for the later conversion of 'choose group' to 'scan groups'.
No functional changes.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250714130327.1830534-13-libaokun1@huawei.com


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