ext4: stop messing with nr_to_write in ext4_da_writepages()

Writeback code got better in how it submits IO and now the number of
pages requested to be written is usually higher than original 1024.
The number is now dynamically computed based on observed throughput
and is set to be about 0.5 s worth of writeback.  E.g. on ordinary
SATA drive this ends up somewhere around 10000 as my testing shows.
So remove the unnecessary smarts from ext4_da_writepages().

Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>