Btrfs: actually call btrfs_init_lockdep

btrfs_init_lockdep only makes our lockdep class names look prettier, thus
it did never hurt we forgot to actually call it. This turns our lockdep
identifier strings from lockdep auto-set #[id] into really pretty
"btrfs-fs-01" or "btrfs-csum-03".

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>