ext4: make ext4_da_map_blocks() buffer_head unaware

ext4: make ext4_da_map_blocks() buffer_head unaware

After calling the ext4_da_map_blocks(), a delalloc extent state could
be identified through the EXT4_MAP_DELAYED flag in map. So factor out
buffer_head related handles in ext4_da_map_blocks(), make this function
buffer_head unaware and becomes a common helper, and also update the
stale function commtents, preparing for the iomap da write path in the
future.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240517124005.347221-11-yi.zhang@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>