ext4: rename 'page' references to 'folio' in multi-block allocator

ext4: rename 'page' references to 'folio' in multi-block allocator

The ext4 multi-block allocator now fully supports folio objects. Update
all variable names, function names, and comments to replace legacy 'page'
terminology with 'folio', improving clarity and consistency.

No functional changes.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Message-ID: <20251121090654.631996-16-libaokun@huaweicloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>