tls: expand use of tls_cipher_desc in tls_sw_fallback_init

tls_sw_fallback_init already gets the key and tag size from
tls_cipher_desc. We can now also check that the cipher type is valid,
and stop hard-coding the algorithm name passed to crypto_alloc_aead.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/c8c94b8fcafbfb558e09589c1f1ad48dbdf92f76.1692977948.git.sd@queasysnail.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>