f2fs: Fix zoned block device support

f2fs: Fix zoned block device support

The introduction of the multi-device feature partially broke the support
for zoned block devices. In the function f2fs_scan_devices, sbi->devs
allocation and initialization is skipped in the case of a single device
mount. This result in no device information structure being allocated
for the device. This is fine if the device is a regular device, but in
the case of a zoned block device, the device zone type array is not
initialized, which causes the function __f2fs_issue_discard_zone to fail
as get_blkz_type is unable to determine the zone type of a section.

Fix this by always allocating and initializing the sbi->devs device
information array even in the case of a single device if that device is
zoned. For this particular case, make sure to obtain a reference on the
single device so that the call to blkdev_put() in destroy_device_list
operates as expected.

Fixes: 3c62be17d4f562f4 ("f2fs: support multiple devices")
Cc: <stable@vger.kernel.org> # v4.10
Signed-off-by: Masato Suzuki <masato.suzuki@wdc.com>
Acked-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>