ext4: provide ext4_do_writepages()

Provide ext4_do_writepages() function that takes mpage_da_data as an
argument and make ext4_writepages() just a simple wrapper around it. No
functional changes.

Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20221207112722.22220-6-jack@suse.cz


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