f2fs: Fix discard bug on zoned block devices with 2MiB zone size

When using f2fs on a zoned block device with 2MiB zone size, IO errors
occurs because f2fs tries to write data to a zone that has not been reset.

The cause is that f2fs tries to discard multiple zones at once. This is
caused by a condition in f2fs_clear_prefree_segments that does not check
for zoned block devices when setting the discard range. This leads to
invalid reset commands and write pointer mismatches.

This patch fixes the zoned block device with 2MiB zone size to reset one
zone at a time.

Signed-off-by: Yonggil Song <yonggil.song@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>