Function put_u16_len_prefixed 

pub fn put_u16_len_prefixed(
    payload: &mut [u8],
    off: usize,
    data: &[u8],
) -> Option<()>

Write a raw u16 length prefix followed by the bytes at payload[off..] (the common [len: u16][data...] framing used by OPEN/CREATE/UNLINK).

Returns None if the data does not fit (> u16::MAX or out of bounds).