btrfs: merge btrfs_find_device and find_device

btrfs: merge btrfs_find_device and find_device

CVE-2019-18885

Both btrfs_find_device() and find_device() does the same thing except
that the latter does not take the seed device onto account in the device
scanning context. We can merge them.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
(backported from commit 09ba3bc9dd150457c506e4661380a6183af651c1)
[ Connor Kuehl: 'metadata_uuid' does not exist yet, so use the
  previously used 'fsid' data member in 'struct btrfs_fs_devices' ]
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>