f2fs: allow direct read for zoned device

This reverts dbf8e63f ("f2fs: remove device type check for direct IO"),
and apply the below first version, since it contributed out-of-order DIO writes.

For zoned devices, f2fs forbids direct IO and forces buffered IO
to serialize write IOs. However, the constraint does not apply to
read IOs.

Cc: stable@vger.kernel.org
Fixes: dbf8e63f

 ("f2fs: remove device type check for direct IO")
Signed-off-by: Eunhee Rho <eunhee83.rho@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>