Btrfs: Make btrfs_device_uuid() return unsigned long

All callers of btrfs_device_uuid() cast its return type to unsigned long.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>