btrfs: move lockdep class setting out of extent_io_tree_init

The per-inode file extent tree was added in 41a2ee75 ("btrfs:
introduce per-inode file extent tree"), it's the only tree type
that requires the lockdep class. Move it to the file where it is
actually used.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>