tls: allocate the fallback aead after checking that the cipher is valid

No need to allocate the aead if we're going to fail afterwards.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/335e32511ed55a0b30f3f81a78fa8f323b3bdf8f.1692977948.git.sd@queasysnail.net


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