btrfs: Use real device structure to verify dev extent

[BUG]
Linux v5.0-rc1 will fail fstests/btrfs/163 with the following kernel
message:

  BTRFS error (device dm-6): dev extent devid 1 physical offset 13631488 len 8388608 is beyond device boundary 0
  BTRFS error (device dm-6): failed to verify dev extents against chunks: -117
  BTRFS error (device dm-6): open_ctree failed

[CAUSE]
Commit cf90d884 ("btrfs: Introduce mount time chunk <-> dev extent
mapping check") introduced strict check on dev extents.

We use btrfs_find_device() with dev uuid and fs uuid set to NULL, and
only dependent on @devid to find the real device.

For seed devices, we call clone_fs_devices() in open_seed_devices() to
allow us search seed devices directly.

However clone_fs_devices() just populates devices with devid and dev
uuid, without populating other essential members, like disk_total_bytes.

This makes any device returned by btrfs_find_device(fs_info, devid,
NULL, NULL) is just a dummy, with 0 disk_total_bytes, and any dev
extents on the seed device will not pass the device boundary check.

[FIX]
This patch will try to verify the device returned by btrfs_find_device()
and if it's a dummy then re-search in seed devices.

Fixes: cf90d884 ("btrfs: Introduce mount time chunk <-> dev extent mapping check")
CC: stable@vger.kernel.org # 4.19+
Reported-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>