btrfs: fix type of parameter generation in btrfs_get_dentry

The type of parameter generation has been u32 since the beginning,
however all callers pass a u64 generation, so unify the types to prevent
potential loss.

CC: stable@vger.kernel.org # 4.9+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>