f2fs: Always enable discard for zoned blocks devices

Zone write pointer reset acts as discard for zoned block
devices. So if the zoned block device feature is enabled,
always declare that discard is enabled, even if the device
does not actually support the command.
For the same reason, prevent the use the "nodicard" mount
option.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>