btrfs: remove pointless key offset setup in create_pending_snapshot()

There's no point in setting the key's offset to (u64)-1 since we never
use it before setting it to the current transaction's ID. So remove the
assignment of (u64)-1 to the key's offset and move the remainder of the
key initialization close to where it's used.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>