ext4: remove check for zero nr_to_scan in ext4_es_scan()

ext4: remove check for zero nr_to_scan in ext4_es_scan()

After converting fs shrinkers to new scan/count API, we are no longer
pass zero nr_to_scan parameter to detect the number of objects to free,
just remove this check.

Fixes: 1db639a9067f ("fs: convert fs shrinkers to new scan/count API")
Cc: stable@vger.kernel.org # 3.12+
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210522103045.690103-2-yi.zhang@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>