f2fs: judge whether discard_unit is section only when have CONFIG_BLK_DEV_ZONED

The current logic, regardless of whether CONFIG_BLK_DEV_ZONED
is enabled or not, will judge whether discard_unit is SECTION,
when f2fs_sb_has_blkzoned.

In fact, when CONFIG_BLK_DEV_ZONED is not enabled, this judgment
is a path that will never be accessed. At this time, -EINVAL will
be returned in the parse_options function, accompanied by the
message "Zoned block device support is not enabled".

Let's wrap this discard_unit judgment with CONFIG_BLK_DEV_ZONED.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>