btrfs: output an error message if btrfs failed to find the seed fsid

[BUG]
If btrfs failed to locate the seed device for whatever reason, mounting
the sprouted device will fail without any meaning error message:

  # mkfs.btrfs -f /dev/test/scratch1
  # btrfstune -S1 /dev/test/scratch1
  # mount /dev/test/scratch1 /mnt/btrfs
  # btrfs dev add -f /dev/test/scratch2 /mnt/btrfs
  # umount /mnt/btrfs
  # btrfs dev scan -u
  # btrfs mount /dev/test/scratch2 /mnt/btrfs
  mount: /mnt/btrfs: fsconfig system call failed: No such file or directory.
        dmesg(1) may have more information after failed mount system call.
  # dmesg -t | tail -n6
  BTRFS info (device dm-5): first mount of filesystem 64252ded-5953-4868-b962-cea48f7ac4ea
  BTRFS info (device dm-5): using crc32c (crc32c-generic) checksum algorithm
  BTRFS info (device dm-5): using free-space-tree
  BTRFS error (device dm-5): failed to read chunk tree: -2
  BTRFS error (device dm-5): open_ctree failed: -2

[CAUSE]
The failure to mount is pretty straight forward, just unable to find the
seed device and its fsid, caused by `btrfs dev scan -u`.

But the lack of any useful info is a problem.

[FIX]
Just add an extra error message in open_seed_devices() to indicate the
error.

Now the error message would look like this:

 BTRFS info (device dm-4): first mount of filesystem 7769223d-4db1-4e4c-ac29-0a96f53576ab
 BTRFS info (device dm-4): using crc32c (crc32c-generic) checksum algorithm
 BTRFS info (device dm-4): using free-space-tree
 BTRFS error (device dm-4): failed to find fsid e87c12e6-584b-4e98-8b88-962c33a619ff when attempting to open seed devices
 BTRFS error (device dm-4): failed to read chunk tree: -2
 BTRFS error (device dm-4): open_ctree failed: -2

Link: https://github.com/kdave/btrfs-progs/issues/959


Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>