MINOR: quic: remove return val of quic_aead_iv_build()

MINOR: quic: remove return val of quic_aead_iv_build()

quic_aead_iv_build() should never fail unless we call it with buffers of
different size. This never happens in the code as every input buffers
are of size QUIC_TLS_IV_LEN.

Remove the return value and add a BUG_ON() to prevent future misusage.
This is especially useful to remove one error handling on the sending
patch via quic_packet_encrypt().

This should be backported up to 2.7.